[ticket/11905] 3.1.0-a1 Migration

Created a migration tips develop tool to help find the migration tree tips so
they can be copied to versions released.

With this, A1 is the only current tree tip (it depends on all other migrations).

PHPBB3-11905
Merge pull request #1767 from EXreaction/ticket/11905

[ticket/11905] 3.1.0-a1 Migration