Fix timezones in booking & wb table filters

[georgmaisser]

Currently, the depicted start endtime depends on the timezone of the user which first triggers cache creation

Todo:

• Every user can see the dates in her own timezone
• Datepicker filter works for all users in their timezone
• Hourlist (weekdays) filter(s) works for all users in their timezone

also see
Wunderbyte-GmbH/Wunderbyte-GmbH#304

[semteacher]

@georgmaisser I think that #77 and #58 have been related to this as well