Group page: display related activity as subtitle

[smadbe]

Overview

A few minor improvements for group header.

Acceptance criteria

• [ ] use the same header size for groups as for items (currently, the group title font is smaller than its subsection

when opening a group:

• [ ] if root_activity_id is not null:

  • [ ] Display as subtitle (same font as in item): "Associated activity: loading..." (or a small spinner in place of "loading...")
  • [ ] fetch the root activity immediately
  • [ ] in case of forbidden error: "Associated activity: not visible"
  • [ ] in case of error: "Associated activity: error while fetching the activity"
  • [ ] in case of success: "Associated activity: name of the activity" where the name is also a link to the activity

• [ ] if root_activity_id is null: display no subtitle