Mentorship requests: add Long/Short term option and creation date

[raluka]

What Github issue does this PR relate to?

Close #92

Changes proposed in this pull request:

• [x] added boolean column longterm to requests and to relationships table
• [x] added creationDate date column to requests table
• [x] In the RequestModal added a switch for longterm, that sends the longterm value to request.js on Submit
• [x] Updated request call to save also longterm and creationDate values into database
• [x] acceptMentorship.js endpoint: createRelationship, and the send email notification function should handle also the longterm value.
• [x] rejectMentorship.js endpoint: the send email notification function (see TODO) should handle also the longterm value.
• [x] cancelRequest.js endpoint: the send email notification function (see TODO) should handle also the longterm value.

What should the reviewer know?

I wasn't able to test locally after latest changed regarding DB were merged, so another end-to-end test would be great

Quality Assurance(QA) Checklist

• [x] The app builds and runs properly

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