Fix where if a user isn't logged into wpcom, go to blank screen

[withinboredom]

When viewing the plans page, it is possible to be logged out of wpcom, this makes sure they go to the right flow depending on whether they're logged in or not by using the jetpack redirector.

Due to Automattic/wp-calypso#22857, we need to control the final destination of these links without requiring a jetpack release.

Changes proposed in this Pull Request:

• Adds the wp-admin user id to the redux state tree
• Adds a reducer to retrieve the wp-admin user id
• Sends the wp-admin user id to the destination (so tracks aliasing works correctly)
• Links go to the jetpack redirector where it makes sense so they go to the right place

Testing instructions:

• View plans page with a free plan
• Open links in new tab to various plans while logged into wpcom, see items auto-added to cart
• Open links in new incognito window to various plans, enter connection flow (plans flow is broken Store: Navigating to checkout for a specific product/site does not prompt customers to log in. wp-calypso#22857)

Proposed changelog entry for your changes:

[withinboredom]

Failing tests look unrelated to this PR

[withinboredom]

fixes #8953

[oskosk]

@withinboredom could you get someone from Growth to approve this one so we can get it cherry-picked to the release branch?

[mattwiebe]

This works and tests as expected, and avoids the blank screen in Calypso in a logged-out state.

Both logged-in and -out flows need improving on the Calypso side, but that's fine. This keeps us in control and we can iterate there without needing to ship a Jetpack release.

[withinboredom]

@oskosk is there some reason this hasn't been merged into the release yet?

[oskosk]

LGTM

[oskosk]

@withinboredom . not other than I've been intermitently offline during the meetup I'm attending during this week. I'm doing it now

[oskosk]

Cherry-picked to branch-5.9 in 35310fd