allow partner selection (pre-search)

[jmckenna]

• goal is to allow users to turn off/on by partner, before executing a search
• maybe have a tree listing all partners in the left panel of the interface, with a checkbox beside each (?)

This has long been asked for by partners, but I don't think it was ever recorded through an issue.

cc @pbuttigieg who has better image mock-up skills than me.

Somewhat related to https://github.com/iodepo/oih-ui/issues/56