Added completion dates and overdue warnings

The project and milestone database tables have been altered. Two fields
were added: complete and completion. These enable the tracking of
late project and milestone completion.

Changes to interface:
-   Overdue labels are now shown on projects, milestones, and tasks that
are not yet complete, but have passed their deadline.
-   Completion labels are shown on projects, milestones, and tasks that
have been completed, with an indication of early/late completion.
-   The labels and dates that are usually shown when clicking the
details button in a task list are now shown alongside the task
instead. The description that was originally shown alongside is now
shown when pressing the details button.
Enabled the assignment of global labels to projects
Added project tags to projects list display