Add hierarchical view of objectives

[jlnat]

Closes #AB1221

User changes

• Users should be able to see a new menu "Top tasks", that displays the tasks at the highest level, and allows navigating to their descendants

Superuser changes

• None

Admin changes

• None

System admin changes

• application.yml or anet-dictionary.yml needs change
• db needs migration
• documentation has changed
• graphql schema has changed

Checklist

• described the user behavior in PR body
• referenced/updated all related issues
• commits follow a repo#issue: Title title format and these 7 rules
• commits have a clean history, otherwise PR may be squash-merged
• added and/or updated unit tests
• added and/or updated e2e tests
• added and/or updated data migrations
• updated documentation
• resolved all build errors and warnings
• opened debt issues for anything not resolved here