[JENKINS-61455] "About Jenkins" accessible with Jenkins.MANAGE (#4569)

* [JENKINS-61455] _About Jenkins_ accesible with Jenkins.MANAGE

_About Jenkins_ management link is now accessible to users with
Jenkins.MANAGE or Jenkins.SYSTEM_READ (as well as the usual
Jenkins.ADMINISTRATOR)

* [JENKINS-61455] Test updated to cover this use case

* [JENKINS-61455] Fix test

* [JENKINS-61455] Fix permissions on linked pages

Co-authored-by: Tim Jacomb <timjacomb1+github@gmail.com>