Paste activity on the activity detail page

Users have reported that they were looking for the activity paste feature inside an already existing activity. We could offer that option with a big warning that any existing content in the activity is lost (possible synergy with #8133). Or we could offer a fake feature that only leads to documentation on the intended way (pasting via picasso).

Also a similar request on the category detail, replacing the content of a category (for editing once and copying to some or all other categories).