Make additional bar when time left with donation exceeding goal

This is a first draft of making another bar appear after the
completed "matched!" bar that will show how much extra we've gotten
beyond the match amount.

Add holiday card to banner at top for next few days

Also, fix a few URLs .

Revert removal of "safe" template tag from blog/news headlines.

Conservancy use HTML entities in their titles.

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.

Apply `futurize --stage-2` Python 2/3 compatibility transformations.

These changes specifically require the use of the "future" library.

Add unique constraint to Staff.username.

Previously had some duplicates for "pono" which were causing
MultipleObjectsReturned errors. A username is generally considered to be unique.

Continuing saga of the sizing chart problems for original shirts

Gildan removed the sizing chart entirely from their website again
after yet another redesign. Originally, we deep-linked into files in
their CDN for the charts, but it appears that in 5c72071 that I
introduced cut-and-paste error on the sizing charts. I cannot find
the original links, but finally I simply decided we'd mirror the
files in our CDN, which is where these now link to.

I suspect that I didn't do this to start for worrying about copyright
infringement, but upon second thought, I think it's very reasonably
fair use for us to distribute these images. We bought a lot of
t-shirts from Gildan and just trying to sell through.

progress bar: Remove display-table vertical align workaround.

Was causing some display glitches on some browsers. Using nested flexbox instead.

progress bar: Drop JS, make mobile friendly, match design to new site.

The existing jQuery UI-based fundraising progress bar used a float layout,
making it hard to adapt for mobile use. Given that there is not interactivity,
I've dropped all the JS and switched to a flexbox layout. This works well
because the bar will stretch to fit the text rather than always maintaining its scale.