Replace time-warp gem with newer, maintained timecop gem

[david-yz-liu]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

In investigating the check_repo_permissions flaky test, I noted that the test had an incorrect "current time", set at July 19 2009, which suggested a previously-mocked time that wasn't correctly reset. Through investigating this, I found the previous gem used for time mocking for tests, time-warp, was not actively maintained, and so I replaced it with the newer timecop gem. I also replaced uses of the Rails test helpers travel_to and travel with uses of timecop.

I enabled Timecop.safe_mode to require the use of do blocks with its mocking, which ensures that the current time is properly reset after the block ends.

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Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
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🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)	X
📦 Dependency update (change that updates a dependency)	X
🔧 Internal (change that only affects developers or continuous integration)

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