#83 - Resource Calendars improvements

[ira-halenok]

• restrict users from deleting a resource calendar in case it is assigned to one of the resource
• fix case when adding a new calendar at the same time when the first calendar is selected, resulted in the incorrect set of time periods. Now, the user is being shown the first calendar on the page's first load. Also, when deleting a calendar, the selection automatically will be changed to the first one. This means that we select the first calendar in the list if the number of calendars changes.