Accept and cancel buttons on Git identity panel

[smashwilson]

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Description of the Change

Instead of writing your username and email directly to git configuration as you type, use an explicit accept button press. While I'm at it, add separate accept buttons to write to repo-local vs. global git configuration, and add more explanation text to ensure that users know what the control is doing.

The "use for this repository" button is disabled in contexts where there is no local repository.

Screenshot or Gif

Applicable Issues

Fixes#2557.

[codecov]

Codecov Report

Merging #2587 (c5cd601) into master (a747a1b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2587 +/- ##
==========================================
- Coverage 93.46% 93.45% -0.01% 
==========================================
Files 237 237 Lines 13231 13234 +3 Branches 1906 1906 ==========================================
+ Hits 12366 12368 +2 - Misses 865 866 +1 

Impacted Files	Coverage Δ
lib/views/git-identity-view.js	100.00% <ø> (ø)
lib/views/git-tab-view.js	86.95% <ø> (ø)
lib/controllers/git-tab-controller.js	86.61% <100.00%> (+0.28%)	⬆️
lib/atom/gutter.js	90.47% <0.00%> (-2.39%)	⬇️

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