Add media relationship: documents borrow

[AJLinn]

Is your feature request related to a problem? Please describe.
I need to add media to a borrow record and there is currently no relationship "documents borrow"

Describe the solution you'd like
I want to add media files to borrow transaction records and link them as such

Describe alternatives you've considered
n/a

Additional context
The link to add media to a borrow exists but the relationship dropdown menu is unclickable (see issue #1667 )

Priority
I would like to have this resolved by ASAP so I can finalize and document a report I'm filing for this borrow.

[Jegelewicz]

Can we get this done by 2018-09-07?

[dustymc]

Done.

#1597