email.py:
- defined `details` list as list literal
user/models.py:
- CheckoutRequest: made date_of_request, date_approved optional
- CheckoutRequest: added code to check availability
- WARNING: I must remember to add the code that checks the MAIN db for availability
errors.py:
- added TsetsedbInsanityError
- began importing all flask and flask extension exceptions I expect to see here for clean importation for error catching
processing/checkouts/spreadsheets.py:
- process_requests: added comments and removed explicit `else` call
data/constants.py:
- added the email messages bunch
user/views.py:
- added imports for errors and constants
- checkout_excel:
    - added exception handling and flash messages for incorrect file type and file import errors
    - added email spam warning from constants
Merge 'feature/check_for_prior_req' into dev