[Web] Clearer captions for domain table columns

Caption the "RL" column "Rate limit" using the existing translated
edit.ratelimit string, and shorten "Max. size of a mailbox" to
"Max. mailbox size" (en-gb only; the key is only used for this column).

Signed-off-by: Moritz Stückler <moritz@bitbetter.de>

data/web/js/site/mailbox.js

@@ -610,7 +610,7 @@ jQuery(function($){
           defaultContent: ''
         },
         {
-          title: 'RL',
+          title: lang_edit.ratelimit,
           data: 'rl',
           searchable: false,
           orderable: false,