Rename submit_form component to page_footer

Submit form was
- a confusing name in itself
- not descriptive, because it also has an optional ‘back’ link

This commit also puts this component in as many pages as possible, stripping
out any hard-coded buttons and links.

It replaces any usage of buttons for ‘back’, because these are links, not
buttons (ie they don’t change any data).

app/assets/stylesheets/components/page-footer.scss

@@ -1,12 +1,10 @@
-.submit-form {
+.page-footer {
 
   margin-bottom: 50px;
 
   &-back-link {
-    @include button($grey-1);
-    padding: 0.52632em 0.78947em 0.26316em 0.78947em;
-    @include inline-block;
-    margin-left: 5px;
+    display: block;
+    margin-top: $gutter;
   }
 
   .button {}

app/assets/stylesheets/main.scss

@@ -33,7 +33,7 @@
 @import 'components/template-picker';
 @import 'components/placeholder';
 @import 'components/sms-message';
-@import 'components/submit-form';
+@import 'components/page-footer';
 @import 'components/table';
 @import 'components/navigation';
 @import 'components/big-number';