Adding multiple transactions to a finance link

[twothreenine]

Currently, when viewing a finance link, you can open modals for adding an existing bank transaction, a financial transaction, and an invoice respectively.
In those modals, you have to click on one of the transactions (its date resp. for invoices, its number) to add it to the finance link. This is a) not very intuitive and b) very cumbersome if you want to add like 10-20 transactions. Also, if you hadn't used finance links previously, you'll have a long list of transactions/invoices without a link that really slows down opening the modal. We currently rely on adding all those transactions to one link via database access (just to get them out of the way).

Suggestion:
Replacing the "linked date" with a checkbox next to each transaction/invoice in the modal, including "select all" checkboxes at the top and the bottom of the list. Placing a "add selected" button next to the "close" button.

I've looked into it, but that's beyond my rails skills. The modals (the _index_... files in here) don't use simple_form_for, which is used by all other forms with a "select all" feature that I could find, and I don't know if a simple_form_for would be applicable here.

This would be a good follow-up to #1064