7398 saved search performance

[scolapasta]

What this PR does / why we need it:
Vastly improves performance for saved search.

Which issue(s) this PR closes:

Closes #7398

Special notes for your reviewer:
The majority of the improvements will be when rerunning a search and parsing through already linked objects. e.g some saved searches, even though they were not adding anything new would take hours; these should now take seconds, maybe minutes.

There is still more work that can be done to improve things here, but feels lower priority at this point.

Suggestions on how to test this:
Try running some saved searches to a) confirm they still work as expected and b) that times are vastly reduced (while the initial link creation should be improved, note that the major improvements will be when rerunning a search and parsing through already linked objects).

Does this PR introduce a user interface change? If mockups are available, please link/include them here:
no

Is there a release notes update needed for this change?:
yes, some suggested db cleanup

Additional documentation:

[coveralls]

Coverage increased (+0.007%) to 19.621% when pulling 89821ab on 7398-saved-search-performance into 56b5b27 on develop.