Fix #9360 regression in content_for
cache_key consults updated_on timestamp if present

- Extract max timestamp retrieval for cache_key
- Update changelog for cache_key changes
Wrong exception is occured when raising no translatable exception
Ensure plugins generated with plugin new can boot the dummy application
Document introduction of the ActionController::UnknownFormat exception
Document extraction of actionpack-xml_parser
Improve docs for UpgradeSignatureToEncryptionCookieStore
Document the switch from memcache-client to dalli
Document rails-observers extraction
Document caveat from 5b3bb61 which fixed handling SCRIPT_NAME from within mounted engine
Merge pull request #9366 from killthekitten/fix-content_for-regression

Fix for #9360 content_for regression
Document sass-rails asset_url deprecation
Improve the CHANGELOG entry

[ci skip]
Merge pull request #9493 from kennyj/fix_wrong_exception

Wrong exception is occured when raising no translatable exception

Closes #8617
Document removal of assets:precompile:primary
Improve the CHANGELOG entry

[ci skip]
Document change in routes using match
Document ActiveRecord::Fixtures becoming ActiveRecord::FixtureSet
change log4r link to most recent, updated location
Document the deprecation of ActionView::RecordIdentifier