Tutor-defined learner limit & automatic unavailability

• Tutors set a maximum number of learners they can handle.
• When the limit is reached, the tutor is automatically marked unavailable.
• Prevent new registrations beyond the defined capacity.

changed milestone to %0.3: Improved User Experience

added To do UX labels

assigned to @DavePikopPokam

marked this issue as related to #160