Move inline comment creation to new DiffInline code

Summary:
Ref T12616. This makes creating inlines use the new code.

Creation and editing is now slightly more consistent in how it uses nodes. This will simplify the next change (replies), which I ran into some trouble with in an earlier iteration.

Note that this (and other changes in the series) allow you to create and edit multiple inlines simultaneously. This is mostly a feature, although I expect we'll need to lock it down a little bit. I have some UI ideas to help avoid errors.

Test Plan: Created inlines on a single line; on a range of lines; on the same line; multiple inlines at the same time.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17893