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