/login route should be modified to check against the conference entered.

[Syntaf]

Additionally, the post into /login will now include the conference. To support this the database should:

• Modify permitted_organizers and permitted_exhibitors so that each entry also contains a foreign key to the corresponding conference.
• Inside login, first check if the conference string is valid inside the conference table
  • If valid, look inside the corresponding permitted table for an email with a matching foreign key
  • then proceed as normal