[server + web] Add settle debts functionality

[Kuchteq]

Server

• Create an openapi GET path at /groups/{id}/settle which will return an array of objects for each member of the group showing how much they owe to one another from the perspective of a given member. It's hard for me to describe it but our figma gives the geneal idea:
  
  If a given member does not owe any money to anyone, they will still be there but the owed section will be missing.
• Create an openapi POST path at /groups/{id}/settle which will receive the names of members that the user wants to reimburse. The handler then adds reimbursement items with the price of the owed money that have the reimbursement field set to true [server] Add reimbursement item field to the schemas #24.

Web

• Create a modal that gets displayed on clicking the settle button. This is visualized on the figma. First do the hardcoded version untill the backend catches up.
• Regenerate the api model and use the client to call the server. Make sure that the items overview in the single group now includes the reimbursement.