Wp-calypso: Navigating to `/settings/:site` or `/sites/:site` results in broken experience

[nylen]

For a user with multiple sites, navigating to /settings/:site or /sites/:site results in a site selector that does nothing upon picking a site:

I experienced this error when mistakenly navigating to /settings/:site instead of /settings/general/:site by editing the URL, then learned that /sites/:site is similarly broken.

Since /settings, /settings/general, and /sites all work without a site, we should probably make sure they do something reasonable with /:site appended to them as well.

[lancewillett]

Good find —

A user-centric note for context, most people probably won't type in URLs like we know how to do, though.

[mikejolley]

This is a duplicate of https://github.com/Automattic/wp-calypso/issues/11543

I submitted an idea for a fix here https://github.com/Automattic/wp-calypso/pull/18053