Fix not being able to submit new topics

flow_definition contains a serialized UUID for discussion's definition_options.
If you've set up Flow a long time ago, that UUID will have been 128-bit.
Depending on when exactly you ran this, propery $hexValue may also be stored
(with an old 128-bit UUID hex representation)

When we merged to 88-bit UUIDs, we've added some B/C code to convert 128-bit
binary values to 88-bit binaries. The hex property was left untouched.

We've recently optimized UUID to not always convert from/to binary anymore,
unless when only needed. This caused $hexValue to be used to convert to
alphanumeric, instead of binary.
Binary was the only format with B/C code for 128-bit to 88-bit, not hex.
This resulted in a 128-bit alphanumeric value being returned.

When, in turn, using that value to find topic definitions (definition_id) in
order to submit a new topic, we failed :)

Change-Id: I65e8e51c5d854e40556c7376291762c98836c200
Merge "Fix not being able to submit new topics"