[#2323] Fixed a typo causing /accounts/relationships to render default relationships. Improved the tests.

Ensured no auxiliary computations (actors list preparation etc.) related to relationships preloading if no user is present (for statuses / accounts / relationships rendering).

Implemented preloading of relationships with parent activities' actors for statuses/timeline rendering. Applied preloading for notifications rendering. Fixed announces rendering issue (preloading-related).

Preloading of follow relations for timeline/statuses rendering (performance improvement). Refactoring.

reel2bits fixture not included as it lacks the Actor fixture for it.

Closes: #1624
Closes: #764

Done via the following command:
git diff fcd5dd25 --stat --name-only | xargs sed -i '/Pleroma Authors/c# Copyright © 2017-2020 Pleroma Authors <https:\/\/pleroma.social\/>'

site_name allow to spoof the origin of the domain and so hacks like:

<!-- served on https://hacktivis.me/tmp/joinmastodon.org.html -->
<meta property="og:image" content="https://hacktivis.me/datalove/img/meme/pleroma/mastodon%2C%20forbidden%20amuse%20yourself.jpeg" />
<meta property="og:title" content="Mastodon: Forbidden Amuse Yourself" />
<meta property="og:site_name" content="joinmastodon.org" />
<meta http-equiv="refresh" content="0; url=http://joinmastodon.org/">

Adds Event support in the same way Video objects are handled, with the
name of the object as message header.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Mastodon API, streaming: Add `pleroma.direct_conversation_id` to the `conversation` stream event payload.

This reverts commit 1ad71592.

Since it had no limit on the number on concurrent processes it OOM killed
instances while rendering hellthreads. When I tried introducing a
concurrency limit with Task.async_stream/manual folds it lead to about 3 times
worse performance on threads larger than 1000 activities (we are talking
30s vs 1.2 minutes), I think this is not worth the about 1.5 times
performance increase on smaller threads when using it.

Otherwise hashtags with word characters other than those allowed in URLs (e.g. Japanese characters) produce hashtag URLs that are invalid.

Needed for pleroma-fe!941

- Try to normalize the activity instead of object wherever possible
- Put the `user` key on non-home timelines as well so bookmarks and
thread mutes are preloaded there as well
- Skip trying to get the user when rendering mentions if the id ==
as:Public or user's follower collection
- Preload the object when getting replied to activities and do not crash
if it's not present

This almost solves the problem of Pleroma hammering the db with a lot
of queries when rendering timelines, the things left are
1. When rendering mentions and the user is not in cache, save it for
later and request all uncached users in one go
2. Somehow get rid of needing to get the latest follow activity to
detect the value of `requested` in a relationship. (create a database
view for user relationship and cache it maybe?)

It was calling CommonAPI.thread_muted? with post author's account
instead of viewer's one.

This reverts commit 2981821d.

NOTE: rewrite the commit msg