Allow filtering accounts by role in API

[nhoening]

A follow-up ticket from the Accounts page issue: allow filtering by account role (e.g. role="<service> subscriber") in the Accounts API (/index). The util function get_accounts already supports passing an account role, so this feature would just require writing data.schemas.account.AccountRoleField using marshmallow.

Originally posted by @Flix6x in #605 (comment)

Addition (NH): I would also enable the account list page to add this filtering. The design question then is how to offer a list of possible account roles to filter for.
Where to get them from: I guess the UI view can get the list of account roles present in the current list of accounts (gotten from /index)
How to show them: A simple dropdown might be okay for now (one option, the default, should be to not filter).