rst: in @mention parser, escape spaces so they don't go to HTML

This eliminates extra spaces around @mentions. Every time mention was followed
by a comma, for example:

@username, have you seen it?

it turned into:

@username , have you seen it?

So an extra space was inserted. It was inserted because otherwise rst parser
might not recognise the markup (i.e. @user1,@user2 is replaced by
**user1**,**user2** — that would be interpreted as <b>user1**,**user2</b>).

See http://docutils.sf.net/docs/ref/rst/restructuredtext.html#character-level-inline-markup