new setting: default value for 'remember me' checkbox

- the default state for the login page's 'remember me' checkbox can now be configured
- adapted the default and vintage theme to consider the new setting
- added documentation for the new setting

tpl/default/loginform.html

@@ -30,7 +30,8 @@
         </div>
         <div class="remember-me">
           <input type="checkbox" name="longlastingsession" id="longlastingsessionform"
-             checked="checked" tabindex="22">
+             {if="$remember_user_default"}checked="checked"{/if}
+             tabindex="22">
           <label for="longlastingsessionform">{'Remember me'|t}</label>
         </div>
         <div>

tpl/vintage/loginform.html

@@ -24,7 +24,9 @@
         </label>
         <input type="submit" value="Login" class="bigbutton" tabindex="4">
         <label for="longlastingsession">
-          <input type="checkbox" name="longlastingsession" id="longlastingsession" tabindex="3">
+          <input type="checkbox" name="longlastingsession"
+                 id="longlastingsession" tabindex="3"
+                 {if="$remember_user_default"}checked="checked"{/if}>
           Stay signed in (Do not check on public computers)</label>
         <input type="hidden" name="token" value="{$token}">
         {if="$returnurl"}<input type="hidden" name="returnurl" value="{$returnurl}">{/if}