Add option to send/edit a message with Ctrl + Enter / Command + Enter

When editing multi-line text this option helps to prevent accidentally
sending a message too early. With this option, Enter just inserts a new
line.

For example, composing programming code in a dev chat becomes much
easier when Enter just inserts a new line instead of sending the
message.

Signed-off-by: Clemens Zeidler <clemens.zeidler@gmail.com>

src/components/views/settings/tabs/user/PreferencesUserSettingsTab.js

@@ -33,6 +33,7 @@ export default class PreferencesUserSettingsTab extends React.Component {
         'MessageComposerInput.autoReplaceEmoji',
         'MessageComposerInput.suggestEmoji',
         'sendTypingNotifications',
+        'MessageComposerInput.ctrlEnterToSend',
     ];
 
     static TIMELINE_SETTINGS = [