Support categories for events.

[ivarshav]

This feature will allow to tag an event with a category form the list of categories that the user created.

This includes:

• Adding a new table and relevant relationship to our DB schema.
• Support the option to add a new category and to choose a category for a new event - FE & BE support.
• Tests that cover this feature.

This issue will focus on BE & DB side.
FE side and most of the tests work will be done in other issue which adi1 leads.

EDR from this issue point of view:

[ivarshav]

Related PRs:
#137
#212
#251

The issue was completed.