Add proposal validation

[trizin]

Fixes #112.

Changes proposed in this PR:

• Removed assert validations from submit_snapshot_accepted_proposals_airtable.js file.
• If a proposal is not valid, DAOBot will set the state to rejected and update the Reason Rejected field.
• Added validations for:
  • One Liner field is not empty.
  • Proposal URL field is not empty.
  • Grant Deliverables field is not empty.
  • Project Lead Full Name field is not empty.
  • Country of Recipient field is not empty.
  • The email address is not empty and valid.
  • The requested USD amount is valid (not over the maximum amount)
• Added verifyEmail function.

Things to do before merging this PR:

• Add Reason Rejected field to the production table