Capture test error messages where appropriate

See merge request pleroma/pleroma!1665

Refactor `add_link_headers/7` -> `add_link_headers/3`

See merge request pleroma/pleroma!1636

Cleanup `Pleroma.Activity` and add more tests

See merge request pleroma/pleroma!1623

Add email change endpoint

Closes #1156

See merge request pleroma/pleroma!1580

Admin config description

Closes #1178 and #1209

See merge request pleroma/pleroma!1611

Fix admin api docs for creating users

See merge request pleroma/pleroma!1660

Fix double quotes in error logs

See merge request pleroma/pleroma!1656

Example:

pleroma: [error] Couldn't fetch ""https://pleroma.soykaf.com/objects/6288a14b-0623-40fc-a26a-0d358f8a11ca"", error: nil

Use Jason for rendering responses

See merge request pleroma/pleroma!1655

Although Jason readme says Phoenix 1.4+ already does it by default, [it actually does it only for new projects](https://github.com/phoenixframework/phoenix/blob/3bfb9f6e900c9a2e31cb95736e2cb5bdad329b61/lib/phoenix.ex#L58-L59)

Mastodon API: URI encode hashtag name in generated URLs

See merge request pleroma/pleroma!1642

Revert "Parallelize template rendering"

See merge request pleroma/pleroma!1654