* Add HTTP signature requirement for served ActivityPub resources

* Change `SECURE_MODE` to `AUTHORIZED_FETCH`

* Add 'Signature' to 'Vary' header and improve code style

* Improve code style by adding `public_fetch_mode?` method

* Remove skip_session! as it is not supported in Rails 5

* Minor cleanup in StreamEntriesController

* Remove redundant mark_cacheable! calls

This will help a great deal with #9377 when a caching reverse proxy is
configured.

Fix #6856
Fix #6951

* Add routing specs for accounts followers and following actions

* Use more restful route naming for public account follow pages

Moves two actions:
- accounts#followers to accounts/follower_accounts#index
- accounts#following to accounts/following_accounts#index

Adds routing spec to ensure prior URLs are preserved.