Finishing off the first iteration on login UI

This makes the following changes:
 - Improve CountryDropdown by allowing all countries to be displayed at once and using PNGs for performance (trading of quality - the pngs are scaled down from 32px to 25px)
 - "I want to sign in with" dropdown to select login method
 - MXID login field that suffixes HS domain (whether custom or matrix.org) and prefixes "@"
 - Email field which is secretly the same as the username field but with a different placeholder
 - No more login flickering when changing ServerConfig (!) fixes https://github.com/vector-im/riot-web/issues/1517

This implements most of the design in https://github.com/vector-im/riot-web/issues/3524 but neglects the phone number login:
![login_with_msisdn](30a921fc-1dfc-11e7-95d1-76f619da1402.png)

This will be updated in another PR to implement desired things:
 - Country code visible once a country has been selected (propbably but as a prefix to the phone number input box.
 - Use square flags
 - Move CountryDropdown above phone input and make it show the full country name when not expanded
 - Auto-select country based on IP

src/components/views/elements/Dropdown.js

@@ -248,13 +248,10 @@ export default class Dropdown extends React.Component {
                 </MenuOption>
             );
         });
-
-        if (!this.state.searchQuery && this.props.searchEnabled) {
-            options.push(
-                <div key="_searchprompt" className="mx_Dropdown_searchPrompt">
-                    Type to search...
-                </div>
-            );
+        if (options.length === 0) {
+            return [<div className="mx_Dropdown_option">
+                No results
+            </div>];
         }
         return options;
     }
@@ -317,7 +314,7 @@ Dropdown.propTypes = {
     onOptionChange: React.PropTypes.func.isRequired,
     // Called when the value of the search field changes
     onSearchChange: React.PropTypes.func,
-    searchEnabled: React.PropTypes.boolean,
+    searchEnabled: React.PropTypes.bool,
     // Function that, given the key of an option, returns
     // a node representing that option to be displayed in the
     // box itself as the currently-selected option (ie. as