removes slugfight from Park model

let it be up to the content editor to find good slug, slugfight in
custom save method is causing problems.
fixes problem when new park couldnt't be saved

because had no PK yet when trying to add neighborhood M2M.
makes area field on park model class nullable
updates requirements
Merge remote-tracking branch 'origin/develop' into develop
resolving merge conflicts
showing the transit search button
brings trailmap legend to front