Add Sticky support for Posts

[bummytime]

Original issue: https://github.com/wordpress-mobile/WordPress-Android/issues/349

No option to set a post as Sticky via the app.

[rachelmcr]

Just a quick note that the new editor (as of alpha-4) does not contain an option to set a post as Sticky.

There's a similar enhancement request for the editor in iOS here: https://github.com/wordpress-mobile/WordPress-Editor-iOS/issues/197

[rachelmcr]

We received another request (in a 4-star app review) for sticky post support:

Guys you should add the 'stick to the front page' button on the app. It's usless if you must log on the pc to do it!

[shaheero]

+1