Save activities using API

[enriquezrene]

In #38 a page to manage activities has been created, we need to implement an API interaction so activities can be actually saved.

Acceptance Criteria

• ActivityService needs to make the proper request to the API
• Visit create-activity.component.ts file to add the required interaction with the above-mentioned service
• Make sure the activity list is reloaded to show the list with the new entry added
• All the added code needs to have at least 80% code coverage

[enriquezrene]

We have deployed a mock server able to simulate the API. In order to implement this ticket you can make the following request:

curl -X POST "http://timetracker-api.azurewebsites.net/activities" -H "accept: application/json" -H "Content-Type: application/json" -d "{ \"name\": \"Activity name\", \"description\": \"Activity description\", \"tenant_id\": \"2b894f3b-03be-4655-abd6-d40e94c23872\"}"