Add event.timestamp with improved range queries

[dcramer]

This adds the ability to find issues which have an event occurrence at a given datetime. It implies date ranges being from (inclusive) => to (exclusive).

Find issues with an event occurrence on Jan 1st 2016:

event.timestamp:2016-01-01

Find issues with an event occurrence on Jan 1st 2016 or Jan 2nd 2016:

event.timestamp:>2016-01-01 event.timestamp:<2016-01-03

Find issues with an event occurrence around Jan 1st 2016 at 05:00 UTC:

event.timestamp:2016-01-01T05:00:00

[codecov-io]

Current coverage is 82.77%

Merging #2628 into master will decrease coverage by -0.45% as of 2dab67e

@@            master   #2628   diff @@
======================================
  Files          864     864       
  Stmts        32664   32738    +74
  Branches         0       0       
  Methods          0               
======================================
- Hit          27186   27100    -86
  Partial          0       0       
- Missed        5478    5638   +160

Review entire Coverage Diff as of 2dab67e

---

Uncovered Suggestions

1. +0.07% via ...try/utils/apidocs.py#414...436
2. +0.07% via ...try/utils/apidocs.py#117...139
3. +0.07% via ...gs/sentry_helpers.py#191...211
4. See 7 more...

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