Update Branding / login label

Everything >= 7.10

• Add video tag field: loop, muted
• Add a multiple videoUrl/MIME type fields in the login page
• Remove login box bg image field
• "Remove header" label should be "Do not use application logo"
• Add a link to know how to override login page: https://doc.nuxeo.com/x/ahQ5AQ. It can be found under the page title 'Branding myBranding' -> 'More' button: "You may prefer to override the whole login page."
• Allow user to edit iframe url from Studio: org.nuxeo.ecm.platform.ui.web.auth.service.LoginScreenConfig#getNewsIframeUrl
• Use 'transparent' color as default for login box bg color
• add login button background color
• documentation/hint for video URLs + MIME type
• in "Main Pages", replace "Add your logo" by "Add Your Application Logo".
• Added support for overriding the login logo.
• background style selector, horizontal/vertical position fields not (in)visible any more, but enabled/disabled.
• text boxes disabled have now a new style: text is lighter, and background is darker
• when creating a new branding, the login button background color is initialized with the default value of the platform.
• use labels or contextual help to explain that:
  • Background image will be displayed first.
  • If available and supported by the browser, the video will be displayed in place of the background image
  • => done in the help message of the videos field
• add contextual help to let user know that he may use 'transparent', '#FFAABB', or 'rgba()'

note: newly added fields should not be primary types to handle xstream deserialization (use Boolean instead of boolean for instance)