Make autocomplete less agressive.

This makes several small changes to the autocomplete widget:
- Autocomplete only replaces input with first dropdown item if input is a
  prefix for the item (this is bug 3444).
- COMMA key no longer triggers autocomplete; TAB and ENTER keys only work.
- Delete key (not backspace) no longer hides autocomplete dropdown.

Testing Done:
- No unit tests currently exist for ui.autocomplete.js
- Reverted delay values.

- Tests: (2 users and 2 groups, John and Johndoe)
- Tested on Windows 8.1 Chrome 38, Firefox 32 and IE11; Linux Mint 17
  (Ubuntu 14) Chromium 37 and Firefox 32
  - type 'John' (wait for dropdown), type 'd', hit TAB/ENTER quickly, does
    not autocomplete since the dropdown hasn't updated due to delay.
  - type 'Jo' (wait for dropdown), hit TAB/ENTER, autocompletes to John.
  - type 'Johnd' (wait for dropdown', hit TAB/ENTER, autocompletes to Johndoe,
    remove 'ndoe', hit TAB/ENTER, autocompletes to John.
  - Comma acts like regular input, does not trigger autocomplete. Starting to
    type another name after a comma behaves normally.
  - TAB acts as normal when autocomplete dropdown not present (jumping through
    elements)
  - ENTER acts as normal when autocomplete dropdown not present (hitting OK
    below input field)

Bugs closed: 3444

Reviewed at https://reviews.reviewboard.org/r/6333/