Add new sizing options for wicket dialogs

Apache Wicket dialogs have some default sizes like small, medium and large. They open a dialog with preset fixed widths and heights. These are unrelated to the current viewport, and do not resize when the viewport size changes.

Add three new sizes for small, medium and large that are expressed in percentages of the viewport. These dialogs should then resize when the viewport size changes. For the percentages, see CHANNELMGR-2352.