Replace Change Comment with transaction custom fields

[btb]

RT has the ability to add custom fields to ticket transactions. Extend this to allow custom fields on asset transactions, this will make the "Change Comment" system redundant.

[btb]

I've started implementing this and the redefinitions required seem nearly as intrusive as the change comments, because in RT, transaction CFs are only implemented for "Update" transactions, i.e. replies and comments.