Implement filtering engine for Ecto resolver

[edisonywh]

With this commit, Ecto resolver can now understand complex filtering like this:

/admin/users?inserted_at[from]=2020-12-30&inserted_at[to]=2020-12-31 /admin/users?id=[gte]60 /admin/users?order_by=[desc]id

But this is only the first step, the next step is to build a UI to go along with it.

This commit also breaks the current search field, which previously searches across multiple fields as specified by users, whereas the new filtering engine expects search per field.

Partially fixes #5