Fix registration after clicking email link

We weren't correctly jumping into the appropriate bit of the registration
flow when coming in from an email link.

 * If we have client secret / sessionId, go straight to registration phase
 * Don't reset server URLs when the server type component tells us its
   initial value
 * Confusingly, pass the custom server URL as 'default server URL' to
   the custom server type, as this is what we want the inital section
   to be based on.

Fixes https://github.com/vector-im/riot-web/issues/8490

src/components/views/auth/ServerTypeSelector.js

@@ -90,7 +90,11 @@ export default class ServerTypeSelector extends React.PureComponent {
             selected: type,
         };
         if (onChange) {
-            onChange(type);
+            // FIXME: Supply a second 'initial' param here to flag that this is
+            // initialising the value rather than from user interaction
+            // (which sometuimes we'll want to ignore). Must be a better way
+            // to do this.
+            onChange(type, true);
         }
     }