[#3213] HashtagsTableMigrator: fault rate allowance to enable the feature (defaults to 1%), counting of affected objects, misc. tweaks.

[#3213] `rescue` around potentially-raising `Repo.insert_all/_` calls. Misc. improvements (docs etc.).

[#3213] Reorganized hashtags cleanup. Transaction-wrapped Hashtag.get_or_create_by_names/1. Misc. improvements.

migration rollback task changes, hashtags-related config handling tweaks, `hashtags.data` deletion (unused).

Add various labels, descriptions and suggestions for all adapter settings and try to use
the same terminology by the service provider.

This setting defaults to false so the relay host will be used in an MX query so
multiple SMTP servers can be used. gen_smtp code states that all records returned from the
MX query are attempted in order and only a permanent SMTP error will stop the client from
attempting other servers in the list. Connection failures, TLS issues, etc will cause it to
try the next host.

If there is no MX record associated with the relay host, it automatically tries connecting to it
directly.

There is really no reason to expose this to end users. The default value is fine for everyone.

[#3213] ActivityPub: implemented subqueries-based hashtags filtering, removed aggregation-based hashtags filtering.

Changed default hashtags filtering strategy to non-aggregate approach.

for MediaProxyWarmingPolicy in ConcurrentLimiter

Pleroma.Web.RichMedia.Helpers & Pleroma.Web.MediaProxy

[#3213] Refactoring of HashtagsTableMigrator. Hashtag timeline performance optimization (auto switch to non-aggregate join strategy when efficient).

The version of Oban we depend on no longer uses ShareLocks, so this
note no longer applies.

Alexander Strizhakov authored Dec 12, 2020 and feld committed Dec 14, 2020

- start oban in mix tasks with empty queues, plugins and crontab
- fix for update_users_following_followers_counts
- fix for removed logo.png
- typo in resend confirmation emails mix task docs
- fix for uploads mix task (start Majic.Pool)
- fix for creating user mix task (start :fast_html app)

Tweaks to OAuth entities expiration: changed default to 30 days, removed hardcoded values usage, fixed OAuthView (expires_in).

Current FedSocket implementation has a bunch of problems. It doesn't
have proper error handling (in case of an error the server just doesn't
respond until the connection is closed, while the client doesn't match
any error messages and just assumes there has been an error after 15s)
and the code is full of bad descisions (see: fetch registry which uses
uuids for no reason and waits for a response by recursively querying a
 ets table until the value changes, or double JSON encoding).

Sometime ago I almost completed rewriting fedsockets from scrach to
adress these issues. However, while doing so, I realized that fedsockets
 are just too overkill for what they were trying to accomplish, which is
 reduce the overhead of federation by not signing every message.
This could be done without reimplementing failure states and endpoint
 logic we already have with HTTP by, for example, using TLS cert auth,
or switching to a more performant signature algorithm. I opened
#2262 for further
discussion on alternatives to fedsockets.

From discussions I had with other Pleroma developers it seems like they
 would approve the descision to remove them as well,
therefore I am submitting this patch.

- revert UserAllowPolicy description
- MRF descriptions order

[#1668] App metrics endpoint (Prometheus) access restrictions

Closes #1668

See merge request !3093