Add the help index page

The index page is a hard-coded list of links to help
content which will exist.

The help index page uses a new set of style rules
which are intended to replace those in support-styles.scss.css
when the existing support pages are removed.
GOV.UK hasn't been "built", we're still building it.

I suppose this commit is a bit meta.
Remove redundant rescue_from calls from HelpController
Update the search entry for the help index page
Make the help page feedback form work with the Report a Problem javascript
Remove the old top border from the help form
Pull out the help form for now
Merge remote-tracking branch 'origin/add-help-index-page'