Split paste create/edit and list views

Summary:
We have this hybrid "create / last few pastes" landing screen right now but I ~never use the list at the bottom and it makes the controller kind of complicated. I want to let you edit pastes too, and this generally simplifies things.

Also makes the textarea monospaced and cleans up the fork logic a bit.

Test Plan: Created, forked pastes. Viewed paste lists. Viewed pastes.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1690

Differential Revision: https://secure.phabricator.com/D3375