Add connector metadata

[sebinside]

To help the user while adding / filling new connectors (aka plattform connections) in the new GUI, connectors should encapsulate more metadata, e.g. by using traits. Helpful would be display name, description and display icon / symbol.

Progress:

• Update REST API
• Rework styling
• Move api type string stuff to requirements card
• Add metadata display to connectors
• Add edit button to connector types
• Add edit button to plugin types
• Add more details button to connectors to display credentials
• Add (small) edit button for each credential key
• Try to declutter everything

[sebinside]

Finished with ea29aaa.