Link to Billing

[jan-krueger]

As requested in #16 I have added the first iteration of this feature. It is still fairly buggy and needs some more work. Right now it just opens the PDF of the invoice when you click the "Retrive Invoice" button.

The biggest issue is figuring out what bill belongs to which item. Right now, I parse the date of the bills and the dates of the items and when there is only one match for each date then that bill is assigned to that item. If there are more matches then I try to compare names but that doesn't always work, so this issue will work really poorly for people who have a lot of bills on a single day. Furthermore, I somehow broke some boxes and you cannot open.

So this PR is still very much a WIP but it already is in a state where it kinda works. So definitely don't merge this just yet.

(When the PDF window opens it is bigger than that but I made it smaller to hide some personal details :))

• [x] Fix issue that some items don't collapse correctly any more
• [x] Improve the way it searches for the correct bill
• [x] Make GET requests to billing page async
• [x] General performance improvements