Improve behavior of inline comment dialog

Summary:
Fixes T2669.

  - Currently, making an inline comment does not focus the textarea. Instead, focus the textarea.
  - Currently, the positioning is kind of buggy. Make it viewport-relative and put the dialog slightly below the inline reticle.
  - Use `JX.Workflow` more and simplify some of the ajax stuff.

Test Plan: Created inline comments.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2669

Differential Revision: https://secure.phabricator.com/D5229