Expand help text on GPX, providing a link to upload GPX files

We know the user is logged in, so we're safe directing right to the upload page.
Convert some remaining en-GB to en-US

Also added alternete dialects to preset search terms

en-US and en-GB are close enough that there's value in having en-GB terms
in the en-US terms list.
Merge pull request #1809 from pnorman/en_dialects

Convert some remaining en-GB to en-US
Merge pull request #1790 from pnorman/gpx_help

Expand help text on GPX, providing a link to upload GPX files
Add terms for fish and chips and petrol
Merge pull request #1815 from pnorman/reddit_synonyms

Add terms for fish and chips and petrol