Saved search improvements [MJ-982]

- Modify return_file helper to accept overrides and return a function
- Modify how jobs are sorted for saved search emails
    - If saved search was sent <= frequency days ago, send saved search for the full frequency
    - Else, send saved search covering all jobs betwen last sent and now.
Merge pull request #812 from DirectEmployers/saved-search-item-filtering

Saved search improvements [MJ-982]