Fix cache clearing of plugins in sharedcontent area

The rendering generated a different cache key, while the
`shared_content:slotname` should only by used for the debugging-comment.
Updated Changelog with sharedcontent plugin fix.
Fix initial south migrations, added missing dependencies.

Cherry picked from: ab6e020d9f4e7b2568e0f23933bb0cad9996bfac
Merge branch 'dev' for 0.9 release

Will not release a separate 0.8.x release,
but rather release a good base for newly planned changes.

Conflicts:
	fluent_contents/rendering.py
Bump version to 0.9, clarify changelog.