Change "Locked" state to "Application submitted"

• Changed "Locked" state to "Application submitted"
• Added a limit of 1 pending or approved application per student with an error message listing conflicts
• Added tests

Still to do (possibly in another MR):

• There is currently no notification to client for withdrawing application
• No support for more than 1 group per person in the same course edition (in course editions where students apply for projects themselves)
• No support for workflows where the client does not approve groups
• Client can approve more than one group
• Some messages in Dutch haven't been updated
• Some places (in the management panel) still refer to locking
• Student conflicts are shown only when attempting to submit an application

Related to #653