Fixed draggable event block alignment

[RichardZhangRZ]

Summary

Fixed realignment of draggable event block on mouse press-down event (see issue #250). The new CSS 'left property is calculated by rounding down the current block ref's left property into a multiple of 20%.

How to Test

1. Add a course or event to the calendar.
2. Add a recurring event that lasts longer than the meeting added in Step 1 and is in conflict with one of its days.
3. Click down on the longer event (but don't move your mouse)
4. The draggable block should be aligned on the left side of the day column.