Merge branch 'main' into glitch-soc/merge-upstream

Conflicts:
- `CONTRIBUTING.md`:
  Upstream file has changed and we're quoting it.
  Ported the changes.
- `README.md`:
  Upstream file has changed but we have a completely different one.
  Kept our version.
- `lib/mastodon/version.rb`:
  Upstream has changed from `tootsuite/mastodon` to `mastodon/mastodon`,
  but we're still `glitch-soc/mastodon`.
  Kept our version.
- `spec/presenters/instance_presenter_spec.rb`:
  Upstream has changed from `tootsuite/mastodon` to `mastodon/mastodon`,
  but we're still `glitch-soc/mastodon`.
  Kept our version.

db/migrate/20200508212852_reset_unique_jobs_locks.rb

@@ -3,7 +3,7 @@ class ResetUniqueJobsLocks < ActiveRecord::Migration[5.2]
 
   def up
     # We do this to clean up unique job digests that were not properly
-    # disposed of prior to https://github.com/tootsuite/mastodon/pull/13361
+    # disposed of prior to https://github.com/mastodon/mastodon/pull/13361
 
     until SidekiqUniqueJobs::Digests.new.delete_by_pattern('*').nil?; end
   end