The assignee page should feature one form and handle submits properly

[shreyas-satish]

The assignee page currently shows one form for each ticket. This is particularly confusing in cases where an order consists of multiple tickets. This situation results in an assignee hunting for an empty form to fill their details.

I propose we show one form for each type of ticket (Item) with a table of currently assigned participants with an edit link. This way, it becomes more obvious to an assignee to fill in their details based on the type of ticket they have.

A related bug on the backend is with how assignee records are created. They are currently created based on the line_item_id submitted by the assignee form. This results in a situation where multiple people filling the same assignee form at the same time, results in only the latest assignee's details being associated with the line item.

When a new assignee is being created, it should be associated with an empty line item. If there are no empty line items, it should result in an error.

[karthikb351]

Recently ran into this issue when I had bought ~20 tickets for the team and sent them an email with the assignment page link and asked them to fill it out.

This resulted in a race condition where most people opened the page around the same time, and all of them filled out the first empty ticket, essentially overriding each other without realising.

Ideally there should be some version number/ auto increment counter on the assignee row to prevent duplicate simultaneous edits and let the second user know they are seeing an out-of-date version of the page.