Fix #6816, fix #6790

* add detailed SMTP settings setup in mastodon:setup

* add localhost SMTP settings setup in mastodon:setup

* SMTP settings setup should exit after successful delivery of test mail

Media attachments are part of the association cache of statuses,
since they are presumed to be immutable. Unless this cache is
cleared manually, the statuses will continue to look like they
have media embedded.

But do not upscale when they are smaller

The default locale is now set by config.

* Some images can cause `convert` to fail, which crashes this whole task

* Add more specific exception

* Ensure the app does not even start if OTP_SECRET is not set

* Remove PAPERCLIP_SECRET (it's not used by anything, actually)

Imports are for internal consumption and the url option isn't even
used correctly, so we can remove the hash stuff from them

* 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

* Fix avatar and header issues by using custom geometry detector

Revert a part of #6508. The file passed to dynamic styles method
was not actually a file, but an instance of Paperclip::Attachment,
which broke all styles by always returning {} from the method.

One problem with GIF avatars was that Paperclip::GeometryDetector
reported wrong dimensions for them, e.g. 120x120 GIF avatar would
for some reason be detected as 120x53. By writing our own geometry
parser, we can use FastImage, which also happens to be faster than
ImageMagick, to detect image dimensions, which are also correct.

Unfortunately, this PR does not implement skipping a `convert`
entirely if the dimensions are already correct, as I found no easy
way to write that behaviour into Paperclip without rewriting the
Paperclip::Thumbnail class.

* Only invoke convert if dimension or format needs to be changed

* Add better CLI prompt

* Add rake mastodon:setup interactive wizard

* Test db/redis/smtp configurations and add admin user at the end

* Test database connection even when database does not exist yet

It is required for ApplicationController.

* Use PNG images in HTML e-mails

* Make webpack use URLs with host so fonts load inside HTML e-mails

Convert this back to a relative URL in the premailer CSS loader
since local requests are quicker

* Improve responsive design

* Add missing PNG icon

* Fix regeneration key not getting expired

* Add rake task to remove old regeneration markers

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates