Syncing with core: Drop PootleSite

core doesn't have a site description, so that is lost. core uses the
settings to specify the site title (in 10-base.conf).

In order to correctly drop this, and aligning with core, the ability
to alter the site title and description through a form in ADMIN has
been removed.
Syncing with core: Drop the goals and tags fields from Store

Apparently no migration is needed since these are not actual fields,
but managers.
Syncing with core: Drop the goals and tags fields from TranslationProject

Apparently no migration is needed since these are not actual fields,
but managers.
Syncing with core: Drop goals and tags

All the goals and tags code included in the other apps is removed. Only
remnants of pootle_tagging app are kept in order to allow running the
migrations.
Syncing with core: Drop remnants of PootleProfile
setup management command no longer allows fresh installs

It also warns after upgrading asking to upgrade to some newer version.
Syncing with core: Drop PootleConfig with dirty hack
Syncing with core: Drop djblets SiteConfiguration database table

Note that this doesn't remove the djblets code because it is required
to provide upgrade paths from ancient Pootle releases.

Since 2.6.0 is meant to be a transitional package the code will be
gone when the next release (possibly 2.7.0) is installed immediatly
after upgrading to 2.6.0.