Improve multisite behavior

[akirk]

This PR attempts to improve the plugin's behavior when running inside multisite environments.

Problem

The Friends plugin was designed for allowing unrelated sites across the web to communicate with each other and establish authentication against each other. They communicate via REST API and represent each other with locally created users. The content for the other sites is downloaded via feeds and assigned to these representative users with a custom post type friend_post_cache.

While this generally works in a multisite context, there are a few things that don't fit the environment well:

• The super admin role has every permission on any site, even if it hasn't been assigned to it. This makes the plugins reliance on user roles fail (for example, it always has the "friend" capability even if it is not a friend of the site).
• It is pointless to create a user to represent the other site, since that other user is on the same multisite (=database) already. This is not only for superflous redundancy reasons. Normally, there is a user created by deriving a username from the URL, but it two friends befriend the same person on the same blog, the same username is shared already resulting a clash. Thus we could equally well use the original user.
• Sub-directory multisites are different once again for discovering if a user's URL resides on the local multisite environment.

Solution

This PR addresses the above in the following ways:

• Super admins no longer have all capabilities when queried for the desired roles
• Multisite users are detected and reused
• It fixes inconsistency for the Friends menu in the admin bar when visiting other sites on the same multisite (for example the friend requests notification remains).
• It adds the ability to rerun the activation routines via the Site Health check in case the Friend roles were not created (see Friends requests not accepting #99).

This has been hard to represent in unit tests but those covering the general behavior still work. I've done a lot of manual testing to ensure that this doesn't break existing flows.

Refs #96, #99.