treewide: Replace xhtml content type with html, remove xhtml workaround

docs/dev/web/controller.rst

...nfig-mode-core/files/lib/gluon/config-mode/www/index.html

...-theme/files/lib/gluon/config-mode/view/theme/layout.html

...-status-page/files/lib/gluon/status-page/view/layout.html

...us-page/files/lib/gluon/status-page/view/status-page.html

...on-status-page/files/lib/gluon/status-page/www/index.html

...gluon-web-osm/files/lib/gluon/web/view/model/osm/map.html

package/gluon-web/luasrc/usr/lib/lua/gluon/web/http.lua