Remove undecided_fields from migrator spec

It's not actually used.
Remove unnecessary function call

This should speed up the migrator a little bit because it cuts out a function
call that happens once per field per record per model.
Migrator Improvements

Resolve overriding migrator defaults better - don't require each
override module to specify things they are just importing from the
default. Make sandiego rules clearer.

Make migration event save an incomplete state by default and overwrite it
with a final state.
Move udf creation logic to FOSS repo
Move udf creation/assignment logic into migrator

Explicitly assign UDFs using preferred syntax
Inline function that is only called once

This abstraction might have been useful when writing the module but at
this point it just slows down a loop that can iterate several hundred
thousand times, without greatly improving readability or DRY.
Rename make_contenttype_relics to match others
Add UDF rules for phillytreemap
Merge pull request #1533 from steventlamb/topic/add_philly_udf_migrator_rules

Topic/add philly udf migrator rules