Prioritize pruning system frames

[dcramer]

This changes the logic in frame trimming to go over system frames first, as they're likely less valuable.

[codecov-io]

Current coverage is 82.58%

Merging #2592 into master will decrease coverage by -0.47% as of 4c1003b

@@            master   #2592   diff @@
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  Files          861     861       
  Stmts        32461   32476    +15
  Branches         0       0       
  Methods          0               
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- Hit          26960   26821   -139
  Partial          0       0       
- Missed        5501    5655   +154

Review entire Coverage Diff as of 4c1003b

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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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[mattrobenolt]

These are going to need to be coordinated, right? getsentry/raven-python#691

Otherwise we may be stripping too much if the client and server aren't agreed on the exact algorithm.

[dcramer]

Im not too worried about coordinating this. The goal of stripping is just to ensure its manageable, and if we're already stripping items that are useful, this wont make it worse (given that its prioritizing app frames).