Uniformized handle data

Summary:
This sets more reasonable values for the object handle fields imo. It's not like I ever want to find out what letter to use and then do `substr($handle->getType(), 0, 1).$handle->getID()` to get `D1` each time I use handles.

Name:

- D1
- T1
- M1
- P1
- etc.

Fullname:

- D1: Something
- T1: Something
- etc.

In addition, this helps me to reasonable prefill Hovercards in case there is no application-specific event listener.

Also deletes `title` and `alternateID` completely. They deserved that.

Test Plan:
Visited places, nothing broke (We only ever used `$handle->getName()` for users and commits).

Tested mail reply handler. Did not test the other way around, but should be fine.

Hovercards broken until D5572 (would love to induce a cyclic dependency)

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5571