[server + web] Edit items in a single group

[Kuchteq]

Server

• Add a get path handler at /groups/{id}/items/{item_id} that would return all the information on a given item. Though, the regular /groups/{id}/items path returns a list of all items with all their properties, in the future, items will get more complicated and this path will provide even more detail.
• Add a put path handler at /groups/{id}/items/{item_id} that the user would call once they make a modification to a given item. Modify the openapi.yml spec file and create the corresponding handler in go calling the database and such.

Web

• Add an on-click on item inside single group that replaces the add item window on the right to edit item. Call a get on /groups/{id}/items/{item_id} to fetch the newest info on the item and populate the editing fields. Hint: while editing an already existing Add Item component might sound like the way to go, you might want to create a seperate component as to avoid overly complex logic inside the initial one. Wisely choosing parts of code to duplicate is OK and readjusting components for multiple purposes may lead to overly complex code structure.
• Make a /groups/{id}/items/{item_id} call once the user finishes editing.