Change references to tootsuite/mastodon to mastodon/mastodon (#16491)

* Change references to tootsuite/mastodon to mastodon/mastodon

* Remove obsolete test fixture

* Replace occurrences of tootsuite/mastodon with mastodon/mastodon in CHANGELOG

And a few other places

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