ENH: Update ExtensionsManager webview when ExtensionsServer url is updated in the settings panel

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21631 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: For consistency, rename "*[Ee]xtensionManager*" methods into "*[Ee]xtensionsManager*"

Note the extra "s" added to the word "Extension".

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21632 3bd1e089-480b-0410-8dfb-8563597acbee