Addressblock: Activate WYSIWYG-Editor on directions and opening hours.

* Changed `directions` and `openingHours` on `Addressblock` from `text/plain` to `text/html` fields.
* Activated tinyMCE widget for those fields.
  * Restricted buttons to bold and link buttons.
* Added upgrade step to convert changed fields on existing `Addressblock` objects to new ContentType.
* Removed `get_opening_hours_as_html` and `get_directions_as_html` and corresponding tests. The new widget saves the input as html, so no conversion is necessary.

@maethu