Create events table in database

[meffmadd]

Is your feature request related to a problem? Please describe.
We have a limited understanding of what is happening and what actions are carried out. We keep logs, but the actual application does not keep track of the actions users take. This will be useful for future features like timed assignments based on fetch (#110).

Describe the solution you'd like
Grader service should log all kinds of events that were carried out. This could include access, git actions, etc.