feat: add Validation for minimum 2 hours

[rajashreemunoli]

For long-term mentor add validation to require 2 hours per month commitment for each mentee number the mentor has mentioned would be able to mentoring.

[lauracabtay]

@dricazenck I’m happy to pick this one up if that can help?

Can I just clarify that the idea to add validation in the Mentorship service (i.e. when creating and updating a mentor, we should check the availability of long term mentor is at least 2 hours per month)?

[lauracabtay]

PR ready for review: #479
Let me know if I have missed anything!

As mentioned in the description, I'm wondering if the check on months should be a bit more sophisticated, i.e. do we expect long term mentors to submit availabilities for a minimum of months in a row? And if so, how many months should be a long-term mentorship minimum)?