[feature] Fetch + display custom emoji in statuses from remote instances (#807)

* start implementing remote emoji fetcher

* update status where pk

* aaa

* tidy up a little

* check size limits for emojis

* thank you linter, i love you <3

* update swagger docs

* add emoji dereference test

* make emoji max sizes configurable

* normalize db.ErrAlreadyExists

docs/configuration/media.md

@@ -39,4 +39,20 @@ media-description-max-chars: 500
 # Examples: [30, 60, 7, 0]
 # Default: 30
 media-remote-cache-days: 30
+
+# Int. Max size in bytes of emojis uploaded to this instance via the admin API.
+# The default is the same as the Mastodon size limit for emojis (50kb), which allows
+# for good interoperability. Raising this limit may cause issues with federation
+# of your emojis to other instances, so beware.
+# Examples: [51200, 102400]
+# Default: 51200
+media-emoji-local-max-size: 51200
+
+# Int. Max size in bytes of emojis to download from other instances.
+# By default this is 100kb, or twice the size of the default for media-emoji-local-max-size.
+# This strikes a good balance between decent interoperability with instances that have
+# higher emoji size limits, and not taking up too much space in storage.
+# Examples: [51200, 102400]
+# Default: 51200
+media-emoji-remote-max-size: 102400
 ```