[STORY] Allow users to hard refresh their problem tracking

[ValerianClerc]

In cases where something breaks with an individual user's problem tracking (as has happened a few times already), we want an easy way to fix this. This comes in the form of allowing users to reset their lastSubmission value, forcing the API to refresh their information from codeforces.

Two parts to this:

• users-microservice: add a route that allows a user to reset only their own lastSubmission
• UI: add a button (on the user's profile? or on the dashboard?) that calls this route