Handle javascript in qute:settings more gracefully.

Fixes #727.

qutebrowser/browser/network/qutescheme.py

@@ -34,6 +34,7 @@ import configparser
 
 from PyQt5.QtCore import pyqtSlot, QObject
 from PyQt5.QtNetwork import QNetworkReply
+from PyQt5.QtWebKit import QWebSettings
 
 import qutebrowser
 from qutebrowser.browser.network import schemehandler, networkreply
@@ -96,6 +97,12 @@ class JSBridge(QObject):
     @pyqtSlot(int, str, str, str)
     def set(self, win_id, sectname, optname, value):
         """Slot to set a setting from qute:settings."""
+        # https://github.com/The-Compiler/qutebrowser/issues/727
+        if (sectname, optname == 'content', 'allow-javascript' and
+                value == 'false'):
+            message.error(win_id, "Refusing to disable javascript via "
+                          "qute:settings as it needs javascript support.")
+            return
         try:
             objreg.get('config').set('conf', sectname, optname, value)
         except (configexc.Error, configparser.Error) as e:
@@ -172,6 +179,14 @@ def qute_help(win_id, request):
 
 def qute_settings(win_id, _request):
     """Handler for qute:settings. View/change qute configuration."""
+    if not QWebSettings.globalSettings().testAttribute(
+            QWebSettings.JavascriptEnabled):
+        # https://github.com/The-Compiler/qutebrowser/issues/727
+        template = jinja.env.get_template('pre.html')
+        html = template.render(
+            title='Failed to open qute:settings.',
+            content="qute:settings needs javascript enabled to work.")
+    else:
         config_getter = functools.partial(objreg.get('config').get, raw=True)
         html = jinja.env.get_template('settings.html').render(
             win_id=win_id, title='settings', config=configdata,