Add the the ability to delete a custom exercise

[urvishp13]

Also

• [x] fix exercises not displaying on regiment page

[urvishp13]

• [x] make "required" tooltip popup when submitting new custom exercise

[urvishp13]

• [x] allow deletion from new exercise page when creating a new exercise
• [x] allow deletion after navigating to custom exercise page from the all-exercises page