Support override of field help text via i18n.

I recently needed a way to provide useful help messages for form fields in the edit/new view. text that relates to the business domain and helps the customer to understand.
I first stumbled over the hint configuration method, but somehow realized that its not used in for form field hints, but for column header tooltips in the index view. Which IMHO is not really useful (whats the use case?) or the name is somehow misleading (since in simple_form form, form field hint texts are called hints :)).

Anyway I found my way to the help configuration method. But I had to repeat and bloat my code, to configure all of the fields with custom help messages. So I thought it would be nice to have a smart way to do this via i18n. This pull request is heavily inspired by the wiki page https://github.com/sferik/rails_admin/wiki/I18n#other-i18n-text.

Objections and suggestions are welcome.