Don't allow saving if there's no pattern title, or if it's a duplicate

[kienstra]

Jen found there's a PHP error if you save a pattern with no title.

And Michael suggested the client side block saving if there's no title or if it's a duplicate.

So this PR does that.

Fixes part of #57 (Deleting title on the edit screen, then saving results in a php error)

How to test

1. Create a new pattern
2. Add some content
3. Remove the title
4. Expected: The 'Update Pattern' button is disabled:

5. Add a title
6. Expected: The 'Update Pattern' button isn't disabled:

[dreamwhisper]

Looks good!

[kienstra]

Thanks a lot, @dreamwhisper!