Rewrite custom user mapping

[simonLeary42]

• added custom mapping IDs to getIDNumsInUse so that they won't be allocated to any other new users
• added warnings
  • custom mapping ID already taken
  • non-csv file in custom user mapping directory ignored
• refactor
  • instead of getUnassignedID wrapping around getNexxtUIDNumber, getNextUIDNumber wraps around getUnassignedID.
  • since UnityGroup uses getNextPIGIDNumber and UnityOrg uses getNextOrgGIDNumber, UnityUser should use getNextUIDNumber.
• change confusing variable name
  • uid_match -> uidnumber (it's not matching anything, and it's not a uid)
  • netid_match -> match (it's not strictly netid, it could be netid or uid)
• style

should be merged after #248

see actual changes

• TODO document custom mappings
• TODO write tests
  • user with custom mapping should get it
  • custom mapped ID should not be given to any other new user

[simonLeary42]

superceded by #286