* Fix error responses in `from` search prefix (addresses mastodon/mastodon#17941)

Using unsupported prefixes now reports a 422; searching for posts from an
account the instance is not aware of reports a 404. TODO: The UI for this
on the front end is abysmal.

Searching `from:username@domain` now succeeds when `domain` is the local
domain; searching `from:@username(@domain)?` now works as expected.

* Remove unused methods on new Error classes as they are not being used

Currently when `raise`d there are error messages being supplied, but
this is not actually being used. The associated `raise`s have been
edited accordingly.

* Remove needless comments

* Satisfy rubocop

* Try fixing tests being unable to find AccountFindingConcern methods

* Satisfy rubocop

* Simplify `from` prefix logic

This incorporates @ClearlyClaire's suggestion (see
https://github.com/mastodon/mastodon/pull/17963#pullrequestreview-933986737).

Accepctable account strings in `from:` clauses are more lenient than
before this commit; for example, `from:@user@example.org@asnteo +cat`
will not error, and return posts by @user@example.org containing the
word "cat". This is more consistent with how Mastodon matches mentions
in statuses. In addition, `from` clauses will not be checked for
syntatically invalid usernames or domain names, simply 404ing when
`Account.find_remote!` raises ActiveRecord::NotFound.

New code for this PR that is no longer used has been removed.

* Prepare Mastodon for zeitwerk autoloader (Rails 6)

Add inflections and rename/move a few classes.

In particular, app/lib/exceptions.rb and app/lib/sanitize_config.rb
were manually loaded while still in autoload paths.

* Add inflection for Url → URL

* Fix movie width and frame_rate returning nil

* Add StreamValidationError and raise

* Fix code style

to_s method of HTTP::Response keeps blocking while it receives the whole
content, no matter how it is big. This means it may waste time to receive
unacceptably large files. It may also consume memory and disk in the
process. This solves the inefficency by checking response length while
receiving.

* Add destination informations to exception on SubscribeWorker and DeliveryWorker.

* Simplify delivery error message.

* Prevent changing Exception type...

* fix typo.

* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)