Remove authority preview from /select_authority

Clicking a search result now takes the user directly to the authority
show page.

Removes the step numbers from the headers as the show page interrupts
the flow and seems pointless to add workarounds in public_body/show just
for this.
Tidy up request/select_authority.html.erb

- Align indentation
- Use named path for form url
- Use %Q() to avoid string escaping
- Use :anchor option to link to help page section
- Use symbols for form tag names
Improve /select_authority search box

- Make it wider
- Add a placeholder
- Translate placeholder and title text
Move “Can’t find an authority” to sidebar

Move the “Can’t find an authority” actions on the
/select_authority page to the right of the search
results