Fix get_sessions_in_range

> I guess this wasn't really different before, but selecting sessions from a single day, by selecting the same date in the from and to fields, is not possible this way. Actually if to is the current date, sessions from today won't be shown. A simple fix would be to just add one day to the value in to here, but since the submitted timestamp could include a time that might not yield the correct result. Another option is to "fix" the timestamp on the client before submitting, i.e. make the time part of the to date always 23:59:59 instead of 00:00:00, which I suppose is the default.

See https://github.com/tnc-ba/strongTNC/commit/0fd15b65a59dfdab72c793d0e85cb224d43a16d7#commitcomment-6448056