fix: correctly assign existing users as institutional mangers

[fdalcin]

Issue #29

This PR aims to fix an issue when trying to assign a user as institutional manager. The issue happened when the user was already assigned as a regular user in the institution. So when trying to make them institutional managers we would receive an error for duplicate keys. The update removes the issue by updating existing records instead of trying to create new ones.

In addition, this PR also updates users to regular assigned users when removing them form institutional mangers. Previously, they would be unassigned from the institution.

How to test

1. Assign users to an institution as regular users (you could use the auto assign feature or manually insert them in the database for now)
2. Visit the institution edit page and select a regular user already assigned to the institution
3. The user should be marked as a manager without issues
4. Try removing them from institutional managers
5. They should still be assigned to the institution but not as managers anymore

Note

No tests were added for now. The entire CRUD need tests, so they will be added in the same ticket.

Note

If a user is assigned to a different institution as a regular user we currently allow them to be assigned to both institutions, that will be updated in a separate ticket (will link it here later)

[arzola]

LGTM, it works as expected