Persist "Show Changeset" and improve path text selection

Summary:
Ref T13513. Currently:

• If you click the "Show Changeset" button, your state change doesn't actually get saved on the server.
• It's hard to select a changeset path name for copy/paste because the "highlight the header" code tends to eat the event.

Instead: persist the former event; make the actual path text not be part of the highlight hitbox.

Test Plan:

• Clicked "Show Changeset", reloaded, saw changeset visibility persisted.
• Selected changeset path text without issues.
• Clicked non-text header area to select/deselect changesets.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21236