Allow terms to contain duplicate sessions when checking sessions.

This change eliminates duplicates in all_sessions_in_terms when checking sessions; this allows specifying metadata terms of the following form, where multiple terms share common sessions:

```
    terms=[
        dict(name='2012-2016', start_year=2012, end_year=2016, sessions=['2012', '2013']),
        dict(name='2010-2014', start_year=2010, end_year=2014, sessions=['2012', '2013']),
    ],
```

Without this modification, billy returns an error (redacted, with extra debug output shown):
```
DBG all_sessions_in_terms [2012, 2013, 2012, 2013]
DBG all_sessions_in_terms [2012, 2012, 2013]
DBG all_sessions_in_terms [2012, 2013]
11:59:54 CRITICAL billy: Error: no session_details for session(s): ['2012', '2013']
```