Use proper MW messages in front-end.

Remove dummy messages from mw.message() function.
Add the message keys used in templates to 'ext.flow.templating' RL
module.

To test, visit a Flow board and in browser console enter:
  mw.flow.TemplateEngine.getTemplate( 'flow_edit_post' )()
  mw.flow.TemplateEngine.getTemplate( 'flow_block_header_edit' )()
etc.

Change-Id: Ib47b9f03d68045d8c448a12abf23a065a0eca9bc
Merge "Use proper MW messages in front-end." into frontend-rewrite