App should use existing draft when creating new message in thread

[sosnovsky]

New draft is created each time when I try to compose new message from thread screen.
App should check if there is existing draft for this thread and use it, instead of creating new one.

[tomholub]

This could be the same as #834 ?

[sosnovsky]

Yes, it looks similar to opening from Drafts screen doesn't allow to edit text, but I think we can leave this issue for second part - use existing draft when composing new message on thread screen, as currently new draft is created on each reply button tap.

[tomholub]

New draft creation should remain disabled even after this PR