Add option to change the size of images/videos in the timeline (#7017)

Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>
Co-authored-by: J. Ryan Stinnett <jryans@gmail.com>
Co-authored-by: Timo K <toger5@hotmail.de>

res/css/views/messages/_MVideoBody.scss

@@ -1,5 +1,5 @@
 /*
-Copyright 2020 The Matrix.org Foundation C.I.C.
+Copyright 2020 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -18,6 +18,6 @@ span.mx_MVideoBody {
     video.mx_MVideoBody {
         max-width: 100%;
         height: auto;
-        border-radius: 4px;
+        border-radius: $timeline-image-boarder-radius;
     }
 }