events: integrated registrations

[BrunoRosendo]

After #55 and #77, we have a good basis for a minimum version of events on our website. However, this version still uses a Google Forms (or any service) URL to handle the registrations externally.

The idea here is to handle all registrations on the website, without the students having to enter another service. Besides having a better user experience, this integration will allow us to have better control of the registrations and add features like mobile/email validation, automatic event reminders, payments (e.g. "course dinners" and deposits/"cauções"), etc.

For starters, we should discuss how we want to implement event registrations and what features we'd like to include for the MVP. Then, we could model the solution (w/ basic endpoints) in this issue and open new ones for more complex features such as the ones mentioned above.