implement user#delete

+ use to remove user from db at end of test
+ refactor out redis key generation

##### for future:

+ add find-by-phone-uuid endpoint & model method
add lookup for user by :phone-uuid

+ phone needs this to load it's own data based on it's UUID
+ might need some sort of account merging in the future in case phone clears/removes it's UUID (only stored locally)
  + fortunately, we have their email address we can do a lookup on
+ code for find-by-phone-uuid is ugly because we don't have an index on phone-uuid->user
+ starting to need better abstractions or organization over database request methods and endpoint request types. I have only one build target, so I might be able to get away with maintaining only one project