consistent behavior of the composer's create button

This pull request keep the create button disabled until all required information are met. 

- [x] updated the `cantSubmitPost` property so that
 - it checks for title when one is mandatory (ie. when creating a new topic, when editing the 1st post and when creating a private message)
 - it checks for at least one user when creating a private message
- [x] removed the test on the title's length when **saving** the draft. I think this should not prevent the draft to be saved. After all, isn't the body the most important part?
- [x] updated the `tabindexes` on the composer view so that it works better when creating a private message
- [x] autofocus the title textfield when editing a topic title using the pencil on the top of the page