Fix calendar z-index and Conpherence/Calendar integration

Summary:
Fixes T7975. Long ago, this element looked like this when you expanded it:

+-------------------+
|     3 4 5 6 7   X |
| 8 9 1 2 3 4 5 +---+
| 6 7 8 9 1 2 3 |
| 4 5 6 7       |
+---------------+

That was why the icon needed a z-index. See T5880. @chad fixed this a while ago so it looks like this:

+---------------+
|     3 4 5 6 7 | X
| 8 9 1 2 3 4 5 |
| 6 7 8 9 1 2 3 |
| 4 5 6 7       |
+---------------+

...but we never stripped the z-index off, causing the bug in T7975.

Also fix some collateral damage from the recent calendar refactoring and the Conpherence widget.

Test Plan:

• Created a new event via Conpherence
• Created a new event normally.
• Browsed a typeahead in Calendar without icons showing through.

Reviewers: lpriestley, chad, btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T7975

Differential Revision: https://secure.phabricator.com/D12639