Add member list pagination and search support to API

This is change includes API support so that we can both a) load the members list page incrementally and b) search for not-yet-loaded members from the members list page.

This changeset contains the following fundamental changes (with related updates to tests):

1. Updated the AttendeeService to take in a list of members to determine eligibility, rather than always returning a list of all members. Required to support pagination.
2. Added pagination support to the member/list endpoint. If a page_size param is sent up, it will only return up to page_size results. The new return format with cursor and has_more is gated based on whether you pass up page_size (for backwards compatibility with the frontend).
3. Added a new /member/search endpoint which accepts a query parameter. This endpoint will return results where the query string is a substring of first, last or email of a member. It also supports pagination.
4. Per discussion with @jeffmay and @dbolson, some refactoring to move more business logic into the MemberService and keep mostly "pure" sqlalchemy queries in the MemberRepo