Under load, multiple backups for a single user could be planned, which
is very expensive.

Define `Settings::BaseController#set_body_classes` so that sub classes
inherit `Settings::BaseController` don't need to define
`#set_body_classes` agein.

* Fix #201: Account archive download

* Export actor and private key in the archive

* Optimize BackupService

- Add conversation to cached associations of status, because
  somehow it was forgotten and is source of N+1 queries
- Explicitly call GC between batches of records being fetched
  (Model class allocations are the worst offender)
- Stream media files into the tar in 1MB chunks
  (Do not allocate media file (up to 8MB) as string into memory)
- Use #bytesize instead of #size to calculate file size for JSON
  (Fix FileOverflow error)
- Segment media into subfolders by status ID because apparently
  GIF-to-MP4 media are all named "media.mp4" for some reason

* Keep uniquely generated filename in Paperclip::GifTranscoder

* Ensure dumped files do not overwrite each other by maintaing directory partitions

* Give tar archives a good name

* Add scheduler to remove week-old backups

* Fix code style issue

* Refactor Export to take an account and know about the export types

* Use Export instance in settings/exports#show

* Allow export of mutes list

* Allow importing of mutes list

* Refactor to use Settings::Exports::BaseController and DRY up exports code

* Add basic coverage for settings/exports controller

* Remove unused @account variable from settings/exports controller

* Add coverage for download export actions

* Remove deprecated `render :text` in favor of `send_data` for csv downloads

* Add model to handle exports

* Use Export class in settings/exports controller

* Simplify settings/exports controller methods

* Move settings/export to more restful routes

* Consolidate webfinger string creation under Account#to_webfinger_s

* Introduce Account#local_username_and_domain for consolidation

Add ability to search accounts by display name