[JENKINS-50447] New design and slim down of login page (#3380)

* [JENKINS-50447] New design and slim down of login page

The change will provide a new designed login page that is using no external deps to prevent any possible malvious javascript.
We still allow the user to use the legacy login page, which has not changed the behaviour.

[JENKINS-50447] Implement error case and use final svg jenkins logo

[JENKINS-50447] remove mock up logo

[JENKINS-50447] Fix feet back from jlong

[JENKINS-50447_squash] Use theme css which only contains values to be overriden. Drop ie js since with the theme css it is not needed anymore

* [JENKINS-50447_squash] Fix expression to evaluate when to show an error and when to simply redirect

* eslint - formating changes and fix offences

* [JENKINS-50447_squash] Use system property to controll whether or not to use the legacy login

* [JENKINS-50447_squash] prevent that the jsessionId appears in the url which should fix most tests

* [JENKINS-50447_squash] Fix cli test by adding pageDecorators magic from layout.jelly

* [JENKINS-50447_squash] remove pageDecorators because they assume existence of js that we do not have

* [JENKINS-50447] New design and slim down of login page

The change will provide a new designed login page that is using no external deps to prevent any possible malvious javascript.
We still allow the user to use the legacy login page, which has not changed the behaviour.

[JENKINS-50447] Implement error case and use final svg jenkins logo

[JENKINS-50447] remove mock up logo

[JENKINS-50447] Fix feet back from jlong

[JENKINS-50447_squash] Use theme css which only contains values to be overriden. Drop ie js since with the theme css it is not needed anymore

[JENKINS-50447_squash] Fix expression to evaluate when to show an error and when to simply redirect

eslint - formating changes and fix offences

[JENKINS-50447_squash] Use system property to controll whether or not to use the legacy login

[JENKINS-50447_squash] prevent that the jsessionId appears in the url which should fix most tests

[JENKINS-50447_squash] Fix cli test by adding pageDecorators magic from layout.jelly

[JENKINS-50447_squash] remove pageDecorators because they assume existence of js that we do not have

[JENKINS-50447_squash] make login page extensible with pagedecorators

[JENKINS-50447_squash] drop legacy support

* Make theme customisable by using a magic package for css and header. Limiting the implemetation of those to one.

* [JENKINS-50447_squash] Creating a specific decorator for login which only returns one instance

* [JENKINS-50447_squash] using rounded corner

* JENKINS-50447 remove legacy code

* [JENKINS-50447_squash] Fix comments from @amuniz about javadoc and uniqueness of css for login

* [JENKINS-50447_squash] Fix click on label span to toggle the select of the checkbox

* [JENKINS-50447_squash] fix comments from @daniel-beck

* [JENKINS-50447_squash] Fix license headers

* [JENKINS-50447_squash] Implement loading/restarting and signup with same design

* [JENKINS-50447_squash] Implement the loading/restarting view and signup with same design as the login page

* [JENKINS-50447_squash] Remove limitation that only one validation error can be shown at a time

* [JENKINS-50447_squash] implement password strength checker

* [JENKINS-50447_squash] Fix different comments from @daniel-beck regarding titles. Further enhanced spacing of label and input

* [JENKINS-50447_squash] Fix restart screen css. Fix comments regarding signup page.

* [JENKINS-50447_squash] fix tabindex flow

* harmonize design
- of checkboxes using jdl styles.
- distance of headers

* [JENKINS-50447] Fix comments from @daniel-beck
- fix some accessibility issues
-- html has now a lang tag
-- define main region
- fix missplaced }
- fix when server down for real that we still show the loading page

* [JENKINS-50447_squash] should never ended up here