Contribute form cleanup

* contribute and add dataset forms are now being handled server side
* dataset name, description and attribution are all editable on submission
* first step with submitting a URL is now being done as a GET; makes things a bit more state-ful
* better formatting of validation error block
* cleaned up and removed some redundant javascript / jinja hybrid code, though form validation is still happening there
* added some more helpful form instructions
* in `meta_master`, increased the field size for `human_name`, `source_url` and `attribution` to 255