[Glitch] Add `/` keyboard shortcut to focus the search field

Port c386c36866 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/features/keyboard_shortcuts/index.jsx

@@ -123,7 +123,7 @@ class KeyboardShortcuts extends ImmutablePureComponent {
                 <td><FormattedMessage id='keyboard_shortcuts.back' defaultMessage='to navigate back' /></td>
               </tr>
               <tr>
-                <td><kbd>s</kbd></td>
+                <td><kbd>s</kbd>, <kbd>/</kbd></td>
                 <td><FormattedMessage id='keyboard_shortcuts.search' defaultMessage='to focus search' /></td>
               </tr>
               <tr>

app/javascript/flavours/glitch/features/ui/index.jsx

@@ -96,7 +96,7 @@ const mapStateToProps = state => ({
 const keyMap = {
   help: '?',
   new: 'n',
-  search: 's',
+  search: ['s', '/'],
   forceNew: 'option+n',
   toggleComposeSpoilers: 'option+x',
   focusColumn: ['1', '2', '3', '4', '5', '6', '7', '8', '9'],