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.