[JENKINS-34674,JENKINS-34675] - Handling of the default update site ID

As detailed by @stephenc:

> So there is `getInstanceOrNull()` which is for use in Jenkins core or
> in code that plugins add that may escape the Jenkins singleton
> lifecycle.
>
> In regular plugins `getInstance()` will never return null as the
> lifecycle does not instantiate them until after there is a Jenkins
> singleton and the plugins will be stopped before there is no
> singleton.
>
> The `getActiveInstance()` is therefore equivalent to `getInstance()`
> and should never have been born.
>
> The only way a plugin can get a `null` from `Jenkins.getInstance()` is
> if you install an `atExit()` handler, use a `PhantomReference` type
> thing, or directly manipulate the servlet container in some way or
> other. A correctly written plugin should unwind any of those things
> when it is stopped or a method bound to the termination lifecycle. If
> there are some cases (for which I cannot anticipate a good coder being
> able to write a correct termination lifecycle method to fix) where the
> code cannot be unhooked... then in those cases the plugin should use
> `getInstanceOrNull()` and know how to respond to the `null` that could
> not be avoided.

[JENKINS-34328] CLI command disconnect-node extracted from core to CLI

[JENKINS-34683] - Do not print stack trace if plugin is missing dependencies

Workflow users may want to catch specific Exceptions without stracktrace in future.

* Add French translation on New Item screen

* Add French translation to Manage Jenkins page

* Add French translation to GlobalToolConfiguration page

* Add missing French translation to Manage Jenkins page

* Correct translations typos and PR remarks

HttpSession listener ExtensionPoint

[FIXED JENKINS-34675] - Prevent hanging of the WebUI if the default Update Site ID cannot be resolved.

Also hardens the Java code a bit.

[JENKINS-34601] Fix help file for node name attribute

[FIX JENKINS-31611] Restrict access to plugin pages

Lithuanian translation update

* [JENKINS-32936] Initial source code modifications

* [JENKINS-32936] Three views are using this approach

* [JENKINS-32936] Conflicts were resolved in a wrong way

disconnect-node extended to accept multiple node names
disconnect-node covered by test-cases

Symbol annotations on core

Move remaining test helper classes from core repo

[FIX JENKINS-15331] Windows file locking workaround

[JENKINS-32874] Improved test coverage of AbstractBuildRangeCommand

[JENKINS-34438] Activate JSHint during the build process

Trying to get a +1 from Daniel

This reverts commit 77555ee8.

Deprecating Computer.cliOnline()

[FIXED JENKINS-33848] - Do not try to resolve empty strings in Items:fromNameList()

[JENKINS-33822] Create a new item using the keyboard