Fix rounding of times when editing availability

[dlemmermann]

Currently the code for creating new availability entries performs a round-up when close to the lower grid line. It should always snap to the upper one (the cell).

[dlemmermann]

The "grid snapping" code is now more refined. When creating a new entry the code will always snap to the earlier time. While dragging the code will snap to the "closer" time, which is either the beginning or the end of the current grid "block".