Include Date Field

[sebaherrera07]

Added a Date Field, where the datepicker lets the admin user select just a date without time.

Notes:

• After needing a Date field on the project I'm working, and seeing there were other people that reported their desire to have it (see Date Field? #741), I decided to contribute to Administrate with this PR 😃

[pablobm]

This looks good to me. I have the slight reservation about handling dates the same way as datetimes (because timezones). However date is one of the native types for ActiveRecord migrations[1], so I think it deserves a field type in Administrate.

Could you please fix the Hound lint warning, before we merge?

Thank you!

[1] https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_column

[pablobm]

Thank you @sebaherrera07!