[Dispatcher] Feature: choose (or be smart about picking) "before" family diff files.

[graphicore]

Currently, the family "Solway" is in google/fonts master git but not available via the google fonts production api (gives "Page not found" at the moment)

Despite of the bad handling of the actual error (see #119) this leaves some questions to consider:

• We may want to choose other diff "before" sources than the Google Fonts API, e.g. the GitHub google/fonts:master or in a (sandboxed? [see Feature: Sandboxed workflow #117]) feature branch workflow maybe another repository/branch to compare to.
• For production google/fonts:mastercould take precedence over the production API if the version in master is newer.
• Should we (and can we actually) also consider the Google Fonts Sandbox API?

Also: DiffBrowsers task should show just previews in this case, just opened #118

[graphicore]

Fixed in 62d5a14