Use `datahub` as param name instead of `domain`

`domain` conflicted with `url_for` generation options and seemed impossible to bypass this feature without manually constructing urls.
Don’t submit search button label in form
Remove unnecessary values from the params hash

So that searches with blank params will end up with matching urls
Merge pull request #955 from theodi/feature-use-nonconflicting-param-name-for-search

Use nonconflicting param name for search filtering