[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,

data/web/lang/lang.en-gb.json

@@ -959,7 +959,7 @@
         "mailbox_defaults_info": "Define default settings for new mailboxes.",
         "mailbox_defquota": "Default mailbox size",
         "mailbox_templates": "Mailbox Templates",
-        "mailbox_quota": "Max. size of a mailbox",
+        "mailbox_quota": "Max. mailbox size",
         "mailboxes": "Mailboxes",
         "max_aliases": "Max. aliases",
         "max_mailboxes": "Max. possible mailboxes",