Release v3.7.0

User-visible changes:
- #808 improved notification email for processed (succeeded or failed)
- #750 metadata for Twitter and OpenGraph (Facebook)
  Mentioning the OSMaxx page should now result in a "preview".
- #792 hardcoded processing time heads-up

Potential bugfixes:
- #806 use a cut-out PBF as input for the splitter (mkgmap/Garmin)
- #775 remove "access denied" page with the "request access" button

Refactorings and maintenance:
- #781 use Python-3-style `super()` calls
- #812 started tracking `*.mo` files (compiled translations)