Category create ideas

[pmattmann]

Followup to #3540

Two ideas:

• New / Empty category, show hint, here you can design the default look of new activities of this category
  solved by Improve admin activity section #4362
• New / Empty category, show copy-option: copy content from other categories of this camp. e.g. someone creates a dedicated hiking-category, one could copy the contentnodes form 'LS' and adjust these.
  OR be able to duplicate an existing category Create category from existing category #4190

[carlobeltrame]

The rest will be tracked in #4190