Docs: Fix the way tabs are declared

Closes #2045

See merge request !2892

Since python doesn't have a way to lock deps for a particlar project
by default, I didn't bother with it. This resulted in mkdocs updating at
some point, bringing a breaking change to how tabs are declared and
broken tabs on docs-develop.pleroma.social. I've learned my lesson
and locked deps with pipenv in docs!5. This MR updates Pleroma
docs to use the new tab style, fortunately my editor did most of it.

Closes #2045

Update AdminFE bundle for 2.1.0 release

See merge request !2889

Mogrifun: Add a line about the purpose of the module.

Closes #1806

See merge request !2888

[#2036] fixed cron worker

See merge request !2886

Fix activity expirations again

See merge request !2866

side_effects: Fix typo on notification

See merge request !2885

Move Accepts and Rejects to the Pipeline

See merge request !2878

CI: Add cmake to build

See merge request !2884

Update frontend

See merge request !2883

Build files: Add cmake

See merge request !2882

Ensure ap_id column in users table cannot be null

See merge request !2875

User deletion issue exploration

See merge request !2879

Improve static_dir documentation

See merge request !2864

Update :welcome settings in description.exs

Closes #2024

See merge request !2843

[#2036] fix job monitor

See merge request !2877