add new tshirt design to /sustainers

fixed width on 'Become a Sustainer' button

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.

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

Does not rename the models.

supporter: Update contact email address.

Mirroring the recent donate changes.

Can't easily get this centered; just move it right.

Move this picture right because I can't get it centered.

Move photos around and don't use Karen photo.

Karen photo was actually the *really old* shirt which isn't available
anymore.

Supporter: js: fade out “Expand All” anchor when all sections expand

The expandable sections can be expanded either one-by-one, or with
the “Expand All” button. Add a counter for each expandable
section (which requires their div's to have 'id' attributes, lest
they be counted in the '__global' section of expandables).

The __global counter will work as advertised if you have no 'id'
attributes on any of your 'expandable-section'-classed div's, but if
you mix a __global without an id with ones that *do* have an id, it's
likely this particular code won't work for that.

Finally, add some documentation which is probably over-documenting
for someone who knows Javascript and jQuery well, but it took me a
while to figure out this code so I felt throwing some notes in there
might be helpful.

Supporter: pictures: get picture-small working & apply throughout.

I pulled this from the `blog-left` style I used to use in blog posts
and created a new style called picture-small. On smaller screen
real estate, Tony's picture was ultimately too big.

Supporter page: apparently, anchor as empty element wreaks havoc.

Javascript code did something weird with this, putting it back the
more obvious way.

Supporter page: Support a “Expand All” link for collapsible sections

I wrap the entire section that has material that can be expanded in a
div with class `expandable-section`. Once doing so, if you provide an
anchor with the class of `expander`, that anchor will be created with
text in the `data-expand-link-text` attribute.

I've also added some CSS to make the link look a certain way, for
good measure.

supporters: Dynamically construct "Read more" links in JS.

This provides better presentation to people without JavaScript,
and makes it easier to author the HTML.

supporter: Kick off 2020 fundraiser. RT#12984

Image is copyright Remy DeCausemaker and licensed under CC BY-SA 4.0.

supporters: Tell people to write <supporters@tix.sfconservancy.org>.

Part of retiring <accounting@sfconservancy.org>.

supporter: Only render the Annual form for upgraders.

This lets us do more with HTML and less with JavaScript, which is always
nice.

supporter: Make a partial for the supporter form.

This commit shouldn't make any substantial changes to the form or its
presentation. This is just cleaning things up as a development aid.

supporter: Add id="renew" to the renewal form.

This provides better behavior for non-JavaScript visitors who use a #renew
link (see last commit).

supporter: Support donors upgrading to supporters.

With a valid URL, a past donor can upgrade to a supporter by donating
the rest of the amount. Django validates the data and, if it passes,
puts it in the form. The JavaScript does most of the work from there.