Use a hash to identify join request on urls
Don't hardcode joinrequest id as the param in tests. refs #1400
Use sql syntax for join_request migration
Generate join request's token on constructor, not on before save

A bit better to have the secret token always valid and not only when the
model is saved.
Also increased the length of the token to the default generated by
urlsafe_base64, because why not.

refs #1400
Merge pull request #56 from mconf/feature1400

Use hash to identify join requests

refs #1400