Add reader manage link to top of P2 and A8c Reader feeds

[davemart-in]

Description

@andrewspittle highlighted that there was no easy way to manage P2s that you had followed from the P2 reader feed. I thought we had removed the button, but it turns out that there never was a button there. :-)

I went ahead and added one to the top of the P2 feed and to the A8c specific feed. I also updated the back button come once you're on the Manage page so it works with all three screens.

Before

After

Testing Instructions

• Apply this PR
• Go to http://calypso.localhost:3000/read/p2
• Click the new "Manage" button
• Click the "Back Button
• Repeat with http://calypso.localhost:3000/read/a8c

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-59897

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-59897&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Async-loaded Components (~111 bytes added 📈 [gzipped])

name                                        parsed_size           gzip_size
async-load-calypso-reader-p-2-main               +116 B  (+3.6%)      +41 B  (+3.4%)
async-load-calypso-reader-a-8-c-main             +116 B  (+3.6%)      +38 B  (+3.1%)
async-load-calypso-reader-following-manage        +62 B  (+0.0%)      +32 B  (+0.1%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.