Fix mentioned user not clickable after posting or editing a comment

The contactsMenu plugin was called on avatar elements from
_postRenderItem, which is called when a new comment is added to the
collection. Due to this contactsMenu was not called when messages were
edited; when a new comment is posted _postRenderItem is called, but at
that time the "mentions" attribute is not filled yet, so "@username" is
not replaced by avatars in the message and thus contactsMenu has no
avatars to be called on.

Calling contactsMenu was moved to a new method, _postRenderMessage,
which is called from _postRenderItem and from the success callback when
fetching the model in _onSubmitSuccess (which replaces "@username" by
avatars in the message after posting or editing a comment).

Fixes #4555

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>