Mode improvements for UI (#3109)

* `--mode=off` now works.

* Add Mode::Off as a persistent CLI option.

* "last" not "auto" as default.

* Commit accidentally unsaved file.

* Whitespace

[ci:skip]

* Mode CLI parse fix

* or offline

* Save mode when it gets changed.

* Fix Offline mode

* Fix up mode operations.

* Make passive default, but not overriding.

* Fix test

* Maybe not everyone wants to run an archive node...

js/src/jsonrpc/interfaces/ethcore.js

@@ -166,7 +166,7 @@ export default {
   },
 
   mode: {
-    desc: 'Get the mode. Results one of: "active", "passive", "dark", "off".',
+    desc: 'Get the mode. Results one of: "active", "passive", "dark", "offline".',
     params: [],
     returns: {
       type: String,
@@ -330,7 +330,7 @@ export default {
     params: [
       {
         type: String,
-        desc: 'The mode to set, one of "active", "passive", "dark", "off"'
+        desc: 'The mode to set, one of "active", "passive", "dark", "offline"'
       }
     ],
     returns: {