Control date overlapping in different enrollments

When two or more enrollments are added to a given profile, Hatstall should prevent or at least warn about overlapped start or end dates. I think it is logical that neither start or ending dates from the different enrollments overlap. Besides, this can cause that the affiliation data will not be properly updated in GrimoireLab dashboards.

For instance, let's imagine there is a profile with two enrollments:
* company: `IBM`
* start date: `1900-01-01`
* end date: `2017-03-25`
* company: `Microsoft`
* start date: `2017-03-25`
* end date: `2100-01-01`

With those values, Hatstall should control that there is an overlapping on `2017-03-25` and ask the user to change one of the two dates, to one day after or one day before that date (For instance, the **start date** for the second company would be `2017-03-26`).

No files, tests, or entry points are named. Start by locating the enrollment date validation and its existing tests, then reproduce the IBM/Microsoft boundary case; done means overlapping start or end dates are prevented or warned about and the behavior is covered by tests.

Written by the indexing model from the issue text.