Replace Riot with Element in docs and comments

This only covers the simple cases of references to issues and repos. More
complex areas, such as deployment scripts, will be handled separately.

Part of https://github.com/vector-im/element-web/issues/14864

res/css/views/avatars/_BaseAvatar.scss

@@ -22,7 +22,7 @@ limitations under the License.
     // different results during full reflow of the page vs. incremental reflow
     // of small portions. While that's surely a browser bug, we can avoid it by
     // using `inline-block` instead of the default `inline`.
-    // https://github.com/vector-im/riot-web/issues/5594
+    // https://github.com/vector-im/element-web/issues/5594
     // https://bugzilla.mozilla.org/show_bug.cgi?id=1535053
     // https://bugzilla.mozilla.org/show_bug.cgi?id=255139
     display: inline-block;

res/css/views/rooms/_RoomSublist.scss

@@ -169,7 +169,7 @@ limitations under the License.
     // that the sublists below them do not jump. However, that leaves a gap
     // when scrolled to the top above the first sublist (whose header can only
     // ever stick to top), so we force height to 0 for only that first header.
-    // See also https://github.com/vector-im/riot-web/issues/14429.
+    // See also https://github.com/vector-im/element-web/issues/14429.
     &:first-child .mx_RoomSublist_headerContainer {
         height: 0;
         padding-bottom: 4px;