Content v2

- Updated home page, tentatively-titled "OpenTrails Tools", with the converter and validator presented side by side per #56.
- Updated copy across the converter flow
- Reduced prominence of "under development" banner per #55. Now only appears on home page. In converter flow, the "steps-tracker" now include an indication that future functionality for transforming trailheads and areas is coming soon.
- Scaffold layout for discrete validator feedback pages set—no copywriting yet, as I want to get this PR submitted and connection here is shaky.
- Sundry other smaller style upgrades.