Consider making it easier to use created_at, modified_at filter parameters

[znatty22]

Since created_at and modified_at use the default marshmallow datetime field, its very hard to pass filter parameter validation. As noted before in #188, Marshmallow has issues deserializing datetime fields bc it doesn't ever hit the logic which uses dateutils parser (which is much better at parsing various formats of datetime).

We should consider using our custom marshmallow datetime field for created_at and modified_at in our base schema.

Additionally, we may want to allow people to search by just dates rather than exact timestamp when using created_at, modified_at filter parameters.