added starting time for simple appointment grid

[merrx]

This PR adds a starting time in the simple mode, addressing the second commit in #596. It leaves the default value at 8AM, but allows for other times.

I am not an expert in vue, so the visual for the NcDateTimePicker could be improved, as it is currently cropped. I tried adding "appendToBody" and increasing the z-index, but some blur or focus-magic from vue prevent the selection, so i removed this part.