Implement SharingActivity

This issue is related to user story #55 and subtasks of that user story #56.

The following features need to be implemented -

[x] Update the Habit class to store attributes "overallProgress" and "user"
[x] Design and implement UI for the sharing screen
[x] Develop backend logic to display the public habits and visual indicators of the people that the user is following
[x] Reroute the user to ProgressActivity when the user clicks on a Habit item
[x] Update UML diagram
[x] Update UI mockups

Closes #56

CMPUT301F21T23 / HabitTracker