eval("") is no-op in IE but window.execScript("") isn't.
So bring the uniformity outside

This was lost in 0f2737f8

While several plugins implemented the interface under the old name, only core was checkcasting it.

At this time the target project is unset so it has no known type and we just have to assume the threshold is valid.

First, display instructions when the user has CONNECT, not necessarily ADMINISTER.
Second, when anonymous users cannot CONNECT, show how to use -jnlpCredentials
(and do not bother showing javaws, since it does not work in this case).

[FIXED JENKINS-16397] Just displaying /asynchPeople constructed the expensive People object merely to decide whether or not to display the “REST API” link.

This change does not necessarily improve total performance, since the avatar is still computed.
But (1) the computation is correctly done in the work thread, not in the HTTP response thread;
(2) the computation is done just once for a given User, which could reduce load when many AJAX checks are done.