Clarify text that not all sponsors are here.

Some sponsors remain anonymous. This text should reflect that.

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.

Support anonymous supporter listings by count.

Remove anonymous listing from the alphabetical supporters list and
instead list them by count found.

Supporters list built dynamically.

Hitherto the supporters list has been committed directly to the static
sponsors/index.html file. This was never ideal and a quick hack to
build Conservancy's supporters list at the beginning of the supporters
program.

With this change, a Django app now exists that dynamically generates the
supporters list.

The database rows must be built from Conservancy's internal Ledger file,
which will be done in a separate script.

Rename file to template.

Since we'll produce dynamic content onto this page going forward, this
apparently needs to be in the templates directory now.