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/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';