259 airtable mentor request form endpoints

[hpjaj]

Description of changes

Why is this feature being added?

The FE has the beginnings of a "Mentor Request Form" in mentorRequest.js. This form will only be accessible to ID.me verified users. This data needs to ultimately be populated in Airtable's in our mentorship airtable.

Associated Issues

• https://github.com/OperationCode/operationcode_frontend/issues/890#issuecomment-370988070
• https://github.com/OperationCode/operationcode_backend/issues/155#issuecomment-327019601

What should your feature do?

• [x] create a POST endpoint that hits Airtables API and populates our mentorship airtable with the passed data:
  • mentee's slack user name
  • an array of service ids
  • an array of skillset names
  • additional details text
  • a mentor's id (or full name?)
• [x] validation that the passed user is verified
• [x] test coverage
• [x] updates our API docs
• [x] creates a GET endpoint that returns the following lists of information from Airtable
  • list of service ids and names
  • list of mentor full names and ids
  • list of skillset ids and names
• [x] Remove the Operationcode::Airtable gem
• [x] Remove any Operationcode::Airtable gem code as it has been stale
• [ ] Set the new Airtable env vars in prod

Issue Resolved

Fixes #259

[hpjaj]

@nagano564 - Tagging you so you can follow this PR