[FIXED JENKINS-27183] Avoid deadlock when using build-monitor-plugin

The original motivation for the fix (to prevent users from logging in
once he's removed from the backend identity database) is legitimate,
but it affected too many users.

So as an escape hatch / non-promoted feature switch, I'm adding this
option to bring back the old behaviour.

[JENKINS-26723] Expose displayName in Remote API

Spotted a HUDSON variable in debian default file

show « check now » button on PluginManager updates and available pages

(cherry picked from commit cf2b02dc)

Conflicts:
	changelog.html

(cherry picked from commit 87ab95e7)

TransientActionFactory should implement ExtensionPoint