Modified UsersController to set cache-cancelling cookie for proxy

Issue #7103.  We want our caching proxy server to cache HTML
pages that Omeka normally makes non-cacheable, but we don't want
authenticated pages to get cached, which could be private.
A cookie named _dpla_no_cache is set so that the proxy can be
told not to cache authenticated pages.
Updated branching instructions in README.md
Changed license file to AGPL, per README.md
Added README-DPLA-core-changes.md
Added version to README.md
Merge branch 'release-v13.2'