Feature: share an activity as unlisted

[cqnykamp]

Request

Split the share button into two buttons: share and share unlisted

1. share always means that the content will be searchable. It must pass classication and accessibility checks to be shared
2. share unlisted means the content will be visible to anyone who has the link. It does not need to pass the checks.

Why

The current sharing process is confusing.

• You can always share something publicly, however,
• Depending on the conditions, it may or may not appear in search results

Let's make the share process more explicit.