Many Fixes.

This MR is a cumulative one with 5 fixes including:

• Allow setting preferred_langauge for a MailingList. (Closes #303 (closed))
• Allow setting empty templates to get around the fact that we don't have a way to message decoration. (Closes #331 (closed), Closes #324 (closed))
• Expose all the digest settings from the REST API in Postorius. (Closes #211 (closed))
• Add a badge of held message and pending subscription requests. (Closes #308 (closed))
• Allow viewing, adding and removing Domain owners so it can be later used to allow them to create lists.
• Allow setting per-list member roster visibility options. (Closes #22 (closed))