Finish Fortnox integration

[RafDevX]

See the existing fortnox branch.

Should use Fortnox API to get a list of accounting accounts, allow Kassörer to select one (default value is whatever is recorded in the budget for that budgetpost), submit the transaction to Fortnox, and then log the assigned ver.nr. in Cashflow.

[hfjallborg]

I'll try working on this, but I need to look through everything to figure out what's going on.

@Lulcaz kanske hugger tag i dig i veckan 😄

[Lulcaz]

Ja, jag behöver också kolla igenom, har inte koll på hur den funkar. Det är @Adasjo som har skrivit den.

[hfjallborg]

@Lulcaz Jag har försökt städa upp koden lite och har pushat mina ändringar (31280db och ddc3619). (Hoppas det är OK!) Jag behöver fortfarande testa med ett klient-ID och ett Fortnox-konto.

[hfjallborg]

The foundation for the client "module" is more or less complete and some endpoints have been implemented (user/company info, accounts, cost centers). I will continue adding more endpoints but it would be good to know what should be prioritized, and how everything should be implemented in the UI. So feel free to just tell me here (or in person) or open sub-issues as you see fit!

[Lulcaz]

I think the UI should have two options. By default, it should show a minimal preview of the expense, simmilar to how the accounting page currently looks. An extra field should be added for the account.

There should be an option to expand the preview into something simliar to the image below, where the accountant can edit the account and add new rows.

All relevant info shoul be able to be gathered from the expense/invoice and the account connected to the budget post at budget.datasektionen.se

[hfjallborg]

In case @Lulcaz (or anyone else) wants to work on this, I have added a Fortnox middleware and a require_fortnox_auth decorator for views (bc78b2d), these will hopefully make everything a bit easier (especially if we want more functionality in the future). The middleware will pass the API client and the user's access token (if possible) to the request object, and the decorator can be used to force a user to be authenticated via Fortnox to access a view, otherwise they will be redirected.