[web + server] Add item UI + expense participants

[Kuchteq]

This is an issue that supersede #7 and extends it.
The desired design is available at https://www.figma.com/design/lzUOvguPdSF8103bGHBnDX/Split?node-id=28-387&t=YZP5CRpQmUWGEy0Q-1

Web

• TextField field for the name of the item
• TextField for price. Next to it there is a currency change button.
• Section for selecting who has paid the expense
• Section for selecting who was participating in that expense
• Attach receipt image section with preview
• Extra info section (skip location and tag for now) - just do the extra notes.
• Create the corresponding state reflecting the choices the user has made
• On save send a POST request to /groups/:id/items

Server

• inside schema.sql add a new join table called expense_participants which will have a foreign key referencing the members table id and a foreign key referencing the given items id.
• modify get /groups/{id} to account for the new field in the items