Also cosmetic changes to make the code clearer

grep -rl '# Copyright © .* Pleroma' * | xargs sed -i 's;Copyright © .* Pleroma .*;Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>;'

* I cretaed a folder 'development'
* I split up the file dev.md into three parts and moved it to this folder
  * index.md
  * authentication_authorization.md
  * mrf.md
* I also moved ap_extensions.md
* I created a new file setting_up_pleroma_dev.md

"Our animations are free to be used to enhance your website."

This reverts merge request !2824

Includes a new mix task: pleroma.database fill_old_hashtags

Currently translated at 87.7% (93 of 106 strings)

Translation: Pleroma/Pleroma backend
Translate-URL: https://translate.pleroma.social/projects/pleroma/pleroma/zh_Hans/

- 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)

Currently translated at 100.0% (106 of 106 strings)

Translation: Pleroma/Pleroma backend
Translate-URL: https://translate.pleroma.social/projects/pleroma/pleroma/uk/

This allows to format Japanese furigana (aka ruby) notation.

Present in XHTML 1.1, HTML 5 and later. Absent in XHTML 1.0, HTML 4 and earlier.

See https://www.w3.org/TR/ruby/

Currently translated at 100.0% (106 of 106 strings)

Translation: Pleroma/Pleroma backend
Translate-URL: https://translate.pleroma.social/projects/pleroma/pleroma/he/

* These are the first small steps for issue 2034 "Reports should send a notification to admins".
* I added a new type of notification "pleroma:report" to the the database manually (a migration will need to be written later)
* I added the new type to the notification_controller
* I made the view return the notification. It doesn't include the report itself (yet)

- fetching activity data
- attachment prefetching
- using limiter to prevent overload

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/2270

Currently translated at 45.2% (48 of 106 strings)

Translation: Pleroma/Pleroma backend
Translate-URL: https://translate.pleroma.social/projects/pleroma/pleroma/zh_Hans/

They can indeed be non-unique.

URI paths are not actually case-insesitive, which caused migration issues
 on a number of databases.

Closes #2188

Currently translated at 24.5% (26 of 106 strings)

Translation: Pleroma/Pleroma backend
Translate-URL: https://translate.pleroma.social/projects/pleroma/pleroma/zh_Hans/