XWIKI-10608 use getXWikiPreference('admin_email') if it is filled

http://jira.xwiki.org/browse/XWIKI-10608
Provide API to programmatically flush rendering cache from components for specified document.
Checkstyle fix
Allow to flush whole rendering cache programmatically
XWIKI-10788: Missing gradient on 'More actions' button in ?viewers
XWIKI-10719: Improve the display when comparing 2 document versions on Flamingo
XWIKI-10719: Improve the display when comparing 2 document versions on Flamingo

- Responsive improvements
XWIKI-10790: White on white labels on extension-diff
XWIKI-10679: Blog subcategories are not well displayed on Flamingo

- Caused because lists are done with margins in Colibri and paddings in Flamingo
XWIKI-10791: Content is overlapping panels area when adding high resolution images
XWIKI-10679: Blog subcategories are not well displayed on Flamingo
XWIKI-5347: Upgrade to VelocityTools 2.0
- Upgraded velocity-tools usage in xwiki-platform
- Using dependencyManagement in xwiki-platform-core pom for assigning velocity and velocity-tools versions
XWIKI-10734: Make Modal Popup's width responsive

* Changes to resources/js/xwiki/widgets/modalPopup.css
XWIKI-10734: Make Modal Popup's width responsive

* Changes to resources/js/xwiki/lighbox/lightbox.css
XWIKI-10734: Make Modal Popup's width responsive

* Improve close position
XWIKI-5347: Upgrade to VelocityTools 2.0
- Reverted unneeded platform dependency management changes since they are handled in the commons parent pom.
XWIKI-10698: Improve the display of the Login form on Flamingo

* Copied the template from webapp/templates/login.vm
XWIKI-10698: Improve the display of the Login form on Flamingo

* Replaced the table with our vertical standard.
* Added Bootstrap specific form classes for input-groups
* Separated ÔForgot usernameÕ from ÔForgot passwordÕ
XWIKI-10698: Improve the display of the Login form on Flamingo
XWIKI-10730: Improve responsive Buttons spacing