Student upload/adding/updating fixes

This MR:

• Returns a proper error message when an email address already exists (#334 (closed))
• Shows this message in the front-end (#334 (closed))
• Adds test for the students API endpoint
• Properly handles duplicate emails when uploading a CSV with students (#298 (closed))
• Allows uploads without a # before the student id (#299 (closed))
• Updates the currently assigned student in the front-end after editing (#334 (closed))
• Fixes selecting and assigning a student with the mouse in the students view

Closes #298 (closed), #299 (closed) and #334 (closed)