Reimbursement form fixes and improvements

[raucao]

Fixes the wrong calculation for the BTC total when adding multiple line items, and adds integration tests to make sure something this facepalm-inducing doesn't happen again.

Other improvements:

• Disable the "add" button on the budget overview, until a core contributor account is connected (so you don't notice after filling in the form and potentially have to do it again)
• Automatically pre-select your own account as recipient, since it's the most likely one