Handle tricky problems to get subtitles working on intro video

First and foremost, the mime type has to be supported by the web
server. Either you have to add a `.vtt` mimetype, *or* just put it
as a `.txt` file. I chose the latter since it doesn't require a
custom Apache configuration.

Second, even if you make it a `.txt` file, using a CDN does not seem
to work. I suspect that maybe CDNs do really weird things with
mimetypes when they server them, or maybe the <track> element just
really doesn't like it when the URL is 301'd. 🤷

Anyway, with these two changes: using a `.txt` file, and hosting the
file locally, the subtitles now work properly!

Note, if you use the `default` attribute in the <track> tag, it'll
turn them on by default. Leaving it off does allow the user to turn
them on in my tests in both Chromium and Firefox.

Sustainer page — note about video; frontpage — wider; add youtube

Link to the Youtube version for browsers where <video> element
doesn't work.

Note on the sustainer page that folks should watch it to learn about
us.

Widen into the big box on front page.

Add holiday card to banner at top for next few days

Also, fix a few URLs .

Remove unnecessary use of "safe" template tag.

This tag marks a variable as not requiring escaping by the template engine,
potentially creating cross-site scripting vulnerabilities, so shouldn't be used
unless absolutely necessary. In these cases, I don't think it's necessary.

Repair menu highlighting, both for main menu and submenus

During the recent improvements made on the website, the menus and
submenu highlighting for selected menus fell into disarray.
Correction requires changes to the CSS and templates both, because
Django doesn't really have a convenient way to set variables in
templates to use to solve this.

There are still a few submenu items highlighting not working even
after this commit. More work needs to be done.

Mailing list subscribe link should go to best we can get.

Ultimately, we'd like the button to be right on the front page, but
for now, at least go the subscribe page.

start Q&A page

edit menu to remove About section and rearrange other items

Rename "supporter" to "sustainer" in templates and URLs.

Does not rename the models.

First-pass implementation of new design for home page.

Uses a combination of CSS changes and Tachyons classes to implement the design.

Adds the non-minified Tachyons CSS library itself and SVG icons from Font Awesome.

news: Refactor press release rendering into a partial.

Same rationale as a few commits ago for blog entries.

blog: Headlines clear floats.

This avoids a situation where multiple summaries have floats that stack on
top of each other vertically, which end up getting far away from the actual
text.

In order to make this work, we had to change the styling of #mainContent on
blog pages (and others with a sidebar). Otherwise, headlines with
class="clear" would clear the sidebar as well.

blog: Refactor entry rendering into a partial.

This helps ensure that entries are rendered consistently wherever they
appear.

frontpage: Reorganize.

* Put news and blogs up front.
* Use 30 days as the cutoff for everything, consistently.
* Allow columns to collapse. Hopefully this is the start of a nicer
view on mobile.
* Reduce shading.

frontpage: Remove announce subscription widget.

Due to subscription abuse, we had to enable a nonce in the Mailman
subscription form, which is non-trivial to reproduce here.

Just wrap the whole thing in the <p>.

I don't know why we needed that subbox anyway.

Previous commit definitely did not work.

validator did not like this:

Element form not allowed as child of element span in this
context. (Suppressing further errors from this subtree.)

So, I moved the id into the form rather than a span.

Be consistent about names of social networks.

Also, a grammar fix.

Actually implement up to 5 blogs in template.

The early change to frontpage.py was not sufficient; the template
obviously needs to make use of those five blog posts as well.

Note the cascading effect here. We only put lots on the front page if
they are recent.

Link to Copyleft Compliance more fully.

We had a link to copyleft.org before, which I remove here, and put a
more general link to the copyleft compliance work.

Wording changes and typo fixes for Supporters.

Update information about who can become a Supporter (small companies
can, too), and make sure they're called "Conservancy Supporters"
throughout.

Place Supporters counter on front & sponsors pages

The number of current supporters now appears on the frontpage and the
sponsors page.