Render custom images in reactions (#11087)

* Add support for rendering custom emojis in reactions

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Include custom reaction short names in tooltips

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Use custom reaction shortcode for accessibility

This uses the shortcode in the following places:

* The aria-label of the reaction buttons
* The alt text for the reaction image

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Remove explicit instantiation of `customReactionName` variable and add types

Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>

* Put custom reaction images behind a labs flag

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Use UnstableValue for finding the shortcode

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>
Signed-off-by: Sumner Evans <sumner@beeper.com>

* Move calculation of whether to render custom reaction images up to ReactionRow

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Make alt text more friendly when custom reaction doesn't have shortcode

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Add test for ReactionsRowButton

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Apply suggestions from code review

Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>

* Don't use Optional

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Fix ReactionsRowButton test

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>
Signed-off-by: Sumner Evans <sumner@beeper.com>

---------

Signed-off-by: Sumner Evans <sumner@beeper.com>
Co-authored-by: Tulir Asokan <tulir@maunium.net>
Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>
Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

res/css/views/messages/_ReactionsRowButton.pcss

@@ -22,6 +22,7 @@ limitations under the License.
     border-radius: 10px;
     background-color: $secondary-hairline-color;
     user-select: none;
+    align-items: center;
 
     &:hover {
         border-color: $quinary-content;