Complete date comparison fixes

This is the completion of the work done quickly in 91ce1b8.
Expose ArrowFields in the admin

Flask-Admin has no built-in support for `ArrowField`s. Getting them to
display is simply a matter of setting the WTForms field type with
`form_overrides`.

Fixes #32
Merge pull request #33 from PyGotham/fix-time-zone-comparisons

Fix time zone comparisons