removed lowercase cast from whitelist and invite

whitelist and invite URLs were needlessly lowercased
Merge pull request #20 from pkit/lower_fix

removed lowercase cast from whitelist and invite
added logic for already used invite codes

each invite should only be used once
requests that try to reuse old invites will be denied

before that fix the requests were partially allowed
Merge pull request #21 from pkit/invite_codes

added logic for already used invite codes