Invoice creation handling

[HrvojeJuric]

Idea

Currently, there is just one button that creates invoices but there should be a background service that creates them each month and a user control where they can select a period and create an invoice for that period validated thoroughly.

Acceptance criteria

• [x] add background service that creates invoices
• [x] add date selection to button for creating invoices
• [ ] add verification of timespan

Implementation

Check MudBlazor for date picker components.

[GabrijelAB]

This is in review because I need to check with @HrvojeJuric how exactly the date selector should allow or deny dates.