Links are relative by default

This way the site also works correctly when deployed to a subdir. See
also 64f684d60615c4df2349740eff6cf525418ca9df
Added missing fields to signup form
Manager host can be customized

I.e. run

MANAGER_HOST="http://localhost:3000" middleman server

to test sign up agains a locally running instance of the manager.
Refactoring: using form helpers
Added client-side validations for signup form
Removed duplicated, unused css file
Some styling fixes for signup form
Signup links preselect correct medium type in signup form