This commit is a highly-refactored and up-to-date version of Fyorl's work
which was part of his Google Summer of Code 2012 project "Attachment
Improvements".
PHPBB3-10929
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
