Feature/save custom attributes

@Smotko I'm not clear on how/why https://reciprocitylabs.atlassian.net/browse/CORE-729 happens.

Custom attribute values should be updated only when updating the CustomAttributable. They should not be updated one at a time (through a separate REST endpoint for example).

If they're updated through the CustomAttributable, using the code here, then the old FTRPs should be deleted, and new ones created. From the way you described the behavior though, it seems that isn't happening. I'm trying to understand why.