WPMU: Disabling sub-site override results in broken UI

Related to #14999

The UI for newly created sites does not reflect what users can actually do, if "sub-site override" is disabled. I _believe_ what is supposed to happen, is that the connection of the primary blog is shared by all other blogs and Jetpack should "just work". Instead, the UI for sub-sites shows a range of broken or wrong links, and attempting to use most features doesn't work.

#### Steps to reproduce the issue

1. Disable "sub-site override" on /wp-admin/network/admin.php?page=jetpack-settings
2. Create a new sub-site as a different user
3. Go to Jetpack dashboard
4. See nonsensical range of entirely broken functionality and layout

#### What I expected

Everything to "just work" using the connection of the main site master user

#### What happened instead

Just click around. It's pretty nuts.

**Screenshots**
If applicable, add screenshots to help explain your problem.

Start by reproducing the steps from the network settings page at /wp-admin/network/admin.php?page=jetpack-settings: disable sub-site override, create a sub-site, and open its Jetpack dashboard. Compare the resulting links, layout, and feature behavior with the expected use of the primary blog connection; done means the sub-site UI and features work consistently in that configuration.

Written by the indexing model from the issue text.