Explorer UI - add support for policies

[thfries]

Hi Ditto team,

please find a new PR that adds support for policies. Along with this, several changes were made:

• Moved the add and delete buttons away from the tables and moved Create/Update/Delete consistently on the details view. This worked out better for CRUD operations including templates and allowing to edit the key of an entity.
• Changed the templates for the connection and removed the modal. Templates are handled similar to definitions in the UI now everywhere
• Added definitions for things and allow now to create sample things instead of empty things only
• Added borders to tables and ace editors
• Added more validations - also for connections - (which is quite painful and causes a lot of code)
• Worked on de-selection on many places
• Improved initialization on changing environments

Looking forward to any feedback or comments.

Thomas