[prep-release-3.1.0-a2] Update version to 3.1.0-a2
[prep-release-3.1.0-a2] Add migration for 3.1.0-a2
[prep-release-3.1.0-a2] Prepare Changelog
[prep-release-3.1.0-a2] Add changelog for alpha2
[prep-release-3.1.0-a2] Update version to 3.1.0-a3-dev in install and constant
Merge branch 'prep-release-3.1.0-a2' into develop

* prep-release-3.1.0-a2:
  [prep-release-3.1.0-a2] Update version to 3.1.0-a3-dev in install and constant
  [prep-release-3.1.0-a2] Add changelog for alpha2
  [prep-release-3.1.0-a2] Prepare Changelog
  [prep-release-3.1.0-a2] Add migration for 3.1.0-a2
  [prep-release-3.1.0-a2] Update version to 3.1.0-a2