Don't delete Store and Directory objects anymore

They'll become obsolete instead, thus allowing to keep units around, which
are used for keeping track of translator's activities or for generating
invoices.

Fixes #269.
Added test for deleting/obsoleting FS stores

Refs. #269.
Docs: rewritten sync_stores/update_stores explanations

Refs. #269.
Docs: explain what's the revision
Docs: fixed some links
Docs: include file in toctree
Tests: removed some invalid tests

It looks like there's some Django bug going on here, and the test failures
are only noise. Also, we'll be replacing these views with the client-side
admin app anyways, so it's safe to get rid of them.