Integrate a Visual Diff

[davelab6]

References:

• http://testmyfont.com
• https://github.com/typefacedesign/document-driven-typedesign
• Add visual diff fonttools/fontbakery#238
• Integrate Marc Foley's gfregression app into a new dashboard page #23

fonttools/fontbakery#1579 said:

@rsheeter has requested today a efficient visual regression tool, and there are a few related tasks for this

• a list of all roll backs in the longest possible period, and the reason why the roll back occurred
• fonts/tools repo split out to its own repo, #1466 mentioned this need
• gfregression turned into diffinator and made suitable for presubmit checking, with no dependencies on flask or FontBakery itself