implement hellthread mitigation

Closes #474

See merge request !588

SetUserSessionIdPlugTest: try again to fix random ci failures

See merge request !579

[#114] Email confirmation

See merge request !546

[#114] Allowed unconfirmed users to authenticate if :account_activation_required is disabled prior to confirmation.
Ensured that no confirmation emails are sent if :account_activation_required is not true. Adjusted tests.

Added tests for `confirm_email` action.

[#413] fix parse mentions

See merge request !577

Fix config.md path in README

See merge request !581

Improve systemd service documentation

See merge request !580

Pleroma-API.md: Import from /wikis/Pleroma-only-api-endpoints

See merge request !571

Use `systemctl` instead of `service` since it's not guaranteed the
latter will be on the system (e.g. Arch does not have it).

unless requested by admin or moderator.

Add a way to use the admin api without a user.

See merge request !576

Per-branch CI cache with force-recompilation of Pleroma

See merge request !574

[#441] Fix characters converted to HTML being picked up by hashtag parser

See merge request !575