Adjustable diff font size

[simurai]

Description of the Change

This PR makes the font size of diffs adjustable based on Atom's editor config. The header font size is set by the UI themes.

Bonus:

• File headers now use the font family from UI themes. Before it was hard-coded.
• Hunk headers now use the font family from the config. Before it was hard-coded.

Alternate Designs

N/A

Benefits

Users can change the font size of diffs in Atom's settings.

Possible Drawbacks

Some might have a different preference.

Applicable Issues

Fixes #1858

Metrics

N/A

Tests

1. Open a diff
2. Press cmd - or cmd +
3. Observe how the code changes the font size, but the file and hunk header stay the same.

Documentation

N/A

Release Notes

N/A

User Experience Research (Optional)

N/A

[codecov]

Codecov Report

Merging #1859 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
+ Coverage   91.15%   91.19%   +0.03%     
==========================================
  Files         196      196              
  Lines       10745    10745              
  Branches     1575     1575              
==========================================
+ Hits         9795     9799       +4     
+ Misses        950      946       -4

Impacted Files	Coverage Δ
lib/git-shell-out-strategy.js	87.52% <0%> (-0.36%)	⬇️
lib/controllers/editor-conflict-controller.js	95.95% <0%> (+1.01%)	⬆️
lib/atom/decoration.js	86.74% <0%> (+2.4%)	⬆️
lib/models/conflicts/side.js	96.87% <0%> (+4.68%)	⬆️

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[simurai]

Let's merge this before the next release, since it could be seen as a regression.