Fixes bug 1069039 - Adjust start times for event assignments

Start times for Event Assignment iCal feeds adjusted to 30
minutes prior to event start time in order to allow staff
sufficient set-up time for events.
It is the correct change. However, let's get picky...

Can you change to use this style:

```python
vevent.add('dtstart').value = (
    event.start_time - datetime.timedelta(minutes=30)
)
```
that way you don't have that massive long thing of just empty space that depends on the length of `"vevent.add('dtstart').value"`.
This is important because right now the line "`[ SPACE ]datetime.timedelta(minutes=30))`" depends on the length of the line above. So, if we some day change the word "dtstart" to "dtend" or we change the variable "vevent" to "ve" then the indentation breaks down and becomes out of alignment which means you need to edit other lines too which makes the diff change things that don't actually are changing. 

Secondly, this is a peculiar little hack. 6 months from now, someone might wonder. Why this?! And why 30min? Why not 45min?!
So, I think you should add a verbose (1-2 lines) comment explaining this. Try to avoid the exact number "30 minutes" in the comment.