Overlapping activities are possible

[Mutant]

It's possible to create overlapping activities. One way to do this is:

• Load the week view
• Create an activity
• Click on the WRMS link to display the WR
• Click back
• The activity that was added will not be displayed (until the screen is refreshed)
• A new activity can now be added over the top of the existing one.

There are possibly other ways to do this.

It should probably:

• Reject overlapping activities on the back-end (and return an error to the front-end if this is attempted)
• Fix the issue where newly created activities are not displayed after clicking back (if this is possible)