Remove public trial fields from dossier and repository default views

As per [specification](https://my.teamraum.com/workspaces/onegov-gever-innovation-session/improvement-proposals/ogip6-umsetzung-offentlichkeitsgesetz/view) the fields `public_trial` and `public_trial_statement` must not be shown any more on the default views (*"Eigenschaften anzeigen"*) for dossiers and repository folders.

This pull request contains the necessary refactorings and changes to implement this requirement:

- Factor out browser ZCML into its own `configure.zcml` in `browser` package (dossier and repository)
- Override the `plone.dexterity` default view for all dexterity content (`IDexterityContent`) with a view and template that allow to define `omitted_fields` that are not shown.
- Default views for documents and dossiers have previously been customized (separately). Remove those views.
- Using this `omitted_fields` property, drop the public trial related fields from dossier and repository folder views
- Customize the default view for documents in order to restore the functionality that the widget for the `file` field is displayed in `NO_DOWNLOAD_DISPLAY_MODE`.
 
Is this special handling no longer necessary? How is the responsible displayed?