Release note remove app

[wen-2018]

If this changeset needs to go into the FXC codebase, please add the WMO and FXC label.

One-line summary

This PR can only be merged after #17240 is deployed.

Significant changes and points to review

• removes release notes app entirely after DB clean up is complete.

Issue / Bugzilla link

#17225 (comment)

Testing

[janriokrause]

Looks good with some minor improvements:

1. After rebasing and the previous PR has landed, please do not forget to git rm bedrock/releasenotes/migrations/0003_delete_productrelease.py.

2. Remove releasenotes.ProductRelease \ from the dumpdata call in bin/export-db-to-sqlite.sh (line 196). It will break the export otherwise.

3. Fix the stale # For release notes. comment on "www.mozilla.org" in _csp_img_src in bedrock/settings/__init__.py, or drop the entry if nothing else needs it. Probably @stevejalim knows best.

4. Follow-up PR after this one merges: Remove django-memoize==2.3.1 from requirements/prod.in, then regenerate prod.txt + dev.txt.