rename icon instead of changing the Javascript

That can take effect on Branchable without rebuilding ikiwiki.deb, and
the shorter name is perhaps nicer anyway.

doc/bugs/Missing_Element.mdwn

@@ -3,4 +3,4 @@ It said the following element was not found:
 Url: <https://ikiwiki.info/wikiicons/email.png>
 <https://ikiwiki.info/ikiwiki.cgi?do=prefs>
 
-> [[fixed|done]], the icon is really called `emaillogin.png` --[[smcv]]
+> [[fixed|done]] --[[smcv]]

underlays/login-selector/ikiwiki/login-selector/login-selector.js

@@ -8,7 +8,7 @@ This code is licenced under the New BSD License.
 var selections_email_large = {
     email: {
         name: 'Email',
-	icon: 'wikiicons/emaillogin.png',
+	icon: 'wikiicons/email.png',
         label: 'Enter your email address:',
         url: null
     }