[ATTACHMENTS] Even more further refactoring

Introduce Encoder plugins Instead of abstract upload and thumb modules

Ported attachment routes. In v3 thumbnail depends on existing attachment
so route updated accordingly.

plugins/VideoEncoder/README.md

@@ -0,0 +1,19 @@
+# FFmpeg plugin for GNU social
+(c) 2020 Free Software Foundation, Inc
+
+This is the README file for GNU social's ActivityPub plugin.
+It includes general information about the plugin.
+
+## About
+
+This plugin adds FFmpeg support to GNU social via PHP-FFMpeg.
+
+Currently it serves as a better performant and quality alternative to resize
+animated GIFs than the ImageMagick plugin. However, it has the downside of
+increasing a little the size of the original GIF images for some conversions.
+
+## Settings
+
+Make sure you've set the `upload_max_filesize` and `post_max_size` in php.ini
+to be large enough to handle uploads if you ever experience some error with
+fetching remote images.