Create swagger definition for activities

[EliuX]

The design to support activities in the time tracker is needed. It should allow to use it this way:

The activities are basically a list of types of tasks the user may be doing while working. They are created dynamically based on the needs of every tenant, so they must have a tenant id.

Therefore the structure of an Activity is:

• id: Unique id of the activity
• name: The by which the activity will be identified
• description: More data about this activity
• tenant_id: Then company it belongs to.

The correspondent actions should be:

• Create
• Read a single and multiple ones
• Update
• Delete

Bear in mind that the search will be done in UI with the list of the gotten activities:

Todo:
A swagger definition must be available in order to access this element