[web + server] Add group create functionality

[Kuchteq]

Web

• Add the page title
• TextField for the name
• Members section with TextField for adding new members
• Inside the members section, after adding the member based on the username make a request to the server at members/:username that should return the DisplayName and id of the user. The user profile photo is not yet implemented, so just do a placeholder there.
• Cover photo for the group section with preview
• Create group button
• Create the corresponding state reflecting the choices the user has made
• On save send a POST request to /groups

Server

• Add a new request handler at members/:username that returns the id and displayName of the user.
• Modify POST request handler at /groups/{id} to account for adding members as well as icon name.