feature: #26 date and time format

This PR tackles #26 (closed) - it grew a bit bigger than initially anticipated.

The date and time representation is everywhere as described in #26 (closed):

The only place I couldn’t fix it was in the input fields, as those are the native browser ones and therefore depend on the locale and representation of the system. To make it more clear what timezone we operate in, we could add a small label to the time input field stating which timezone is used, as this is the only place where we don’t show the z now. WDYT?

Notable changes:

• Date-extensions removed: They were not needed and had a small bug (toLocaleTimeString returns 6:15:23 PM depending on your locale, resulting in 6:15: when executing the Date.toShortTimeString-extension).
• (new) DateTimeConverters adapted/rearranged/introduced: I wanted to keep the data we pass around in its original format and only adapt the actual, final representation in the components. It feels a bit messy now - what’s your impression?