Assume unknown db column types are un-searchable

Created by: gracewashere

Closes #178 (closed)

Problem:

If users have a database set up with unknown column types, Administrate treats those columns as a string. This can cause bugs when users try to search, because the unknown database column might not support the string search operator.

Solution:

• Add a searchable option to all field types, which specifies whether it is one of the resource's plaintext search fields.
• For unknown db column types, give the field a default option of searchable: false.