[JENKINS-31661] Check the root url (Admin monitor + Wizard) (#3082)

* [JENKINS-31661] Add an administrative monitor to check the root url configuration
- add the monitor + test

* - corrections requested by Oleg in code review

* - add a field to fill during SetupWizard for the rootUrl (default value using javascript but editable)
- WIP : missing the translation

* - revert whitespace change

* - little refactoring of js methods

* - translation added

* - modification requested during PR by Jesse

* - renaming proposed by Keith

* - rework to have a new page in the SetupWizard for the root URL

* Correct relevant non-conflicting merge changes

* - in case of skip => also skip the instance configuration
- terminate => finish (+ uppercase)
- replace double single quotes
- add help message in the wizard to inform admins
- correct the installState serialization problem
- update working (admin monitor + wizard)

* address Daniel's comment
- skip all steps will show two messages instead of one (like before when clicking on skip twice)
- skip => not now
- remove margin and adjust padding for the wizard forms
- add the "if possible"

* address Arnaud's comment on the sauter vs passer

* address Liam's comment

* correct previous commit typo

* correct previous commit typo (2)

* add blank end line

* address James' comment

* address Daniel's comment

* address Devin's comment
- implement error management in the wizard
- provide different message in the monitor in case of null / invalid url
- add an helper to validate URL (even with _ in domain)
- add @Restricted

* - also update the monitor test to be consistent with its new behavior

* address Devin's comments

* correct header files

* include relevant classes for the url validation from commons-validator:1.6

* Move Apache classes in a more standard package