Add site preference for standard subjects, including support for Re:

Template format similar to reply_key is used.  Tokens supported include:

%{site_name} - email site name preference
%{optional_re} - "Re: " if it is a reply (not first post), "" otherwise
%{optional_pm} - "[PM] " if it is a private message, "" otherwise
%{topic_title} - topic title

Default subject is: %{optional_re}[%{site_name}] %{optional_pm}%{topic_title}

If preferred, %{optional_re} could be omitted from the default subject.

optional_re and optional_pm are backed by subject_re and subject_pm locale translation strings.

Having upgrade my Mac, my local Discourse test system is no longer functional, and my attempt to spin up a Digital Ocean test system failed, so there is currently no tests.

Hopefully someone can provide some appropriate tests to go along with this patch.