Improve systemd service documentation

See merge request pleroma/pleroma!580

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

See merge request pleroma/pleroma!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).

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

Use "~" as a route namespace symbol on FE

See merge request !530

update frontend

See merge request !573

SetUserSessionIdPlugTest: try to fix random ci failures

See merge request !570

Instance/Runtime static plug

See merge request !567

This allows to set-up an arbitrary directory which overrides most of the
static files: index.html static/ emoji/ packs/ sounds/ images/ instance/
favicon.png.

If the files are not present in the directory, the bundled ones in
priv/static will be used.

2018 12 17 update frontend

See merge request !565

Fix uploads test.

See merge request !564

Fix update_profile failing due to OAuth token record in assigns instead of string

Closes #459

See merge request !561

[#449] fix text field

See merge request !563

Captcha

See merge request !550

Fix tagging problems for existing instances.

See merge request !562

Add tests for Mix tasks

See merge request !555

Upgrade to Phoenix 1.4

Closes #451

See merge request !560