Restore enableWebfonts pref and remove uls-enable

This patch restores the enableWebfonts preference, stored inside
the uls-preferences blob. It existed as 'enable-webfonts' in
the past and was removed in
If735a733717596fae03042c5e277bd538bd8501f

Each wiki can be configured to load the fonts by default
using the new global variable $wgULSWebfontsEnabled.
Its default value is true (to load fonts).

This also removes the preference 'uls-enable', recently added in
I71b70d8ee7c3cad7f49b32e5dc494ef4fc1bdb2f

The initialization of ext.uls.webfonts.js is changed as well,
so that minimal webfonts JS library code is loaded,
and the rest is loaded only if a user requests it.

Bug: 60304
Change-Id: I49e812eae32266f165591c75fd67b86ca06b13f0

tests/browser/features/support/pages/panel_page.rb

@@ -10,7 +10,6 @@ class PanelPage
 
 	div(:language_settings_dialog, id: "language-settings-dialog")
 	div(:panel_display, id: "display-settings-block")
-	div(:panel_input, id: "input-settings-block")
 	button(:panel_fonts, id: "uls-display-settings-fonts-tab")
 	button(:panel_language, id: "uls-display-settings-language-tab")
 
@@ -24,6 +23,8 @@ class PanelPage
 	button(:panel_disable_input_methods, class: "uls-input-toggle-button")
 	button(:panel_enable_input_methods, class: "uls-input-toggle-button")
 
+	checkbox(:webfonts_enable_checkbox, id: "webfonts-enable-checkbox")
+
 	select_list(:panel_content_font_selector, id: "content-font-selector")
 	select_list(:panel_interface_font_selector, id: "ui-font-selector")
 
@@ -69,6 +70,9 @@ class PanelPage
 			return ( top < viewportBottom && top >= viewportTop )" )
 	end
 
+	def webfonts_library_loaded
+		@browser.execute_script("return( $( 'body' ).data( 'webfonts' ) !== undefined )")
+	end
 
 	private
 	def font(selector)