Fix failing specs

Before I started refactoring, I wanted to make sure that the tests on master were green.

Here's my attempt at doing so.

Things I'd like to know:

1. Duration? This seems like  it doesn't have a field in the database, or gets inferred as nil? I've defaulted it to 30
2. Twitter  handle also defaults to nil, so I've coerced it to an empty string for the create_activation_code
3. Because Timezones didn't default, it was causing most of the availability code to explode. I've made it so the table defaults to UTC, but there's probably a better way with TZInfo configuration?
4. I've removed a couple tests because the code they were testing didn't exist anymore. Let me know if I should put these back and just pend them out?
5. I migrated all the mentor/mentee specs and factories into user. Is there supposed to be different models for each of these?

Zee