Decouple members and users resources

[enricostano]

Due to some design restriction decided when TO users could only pertain to one time bank, the User and Member resources were coupled and often used interchangeably. Now that we want to introduce the concept of "discover a time bank near to you" and multiple memberships we need to refactor and break this coupling.

This PR is related to https://github.com/coopdevs/timeoverflow/pull/250, but independent. Although would be nice to merge this one first.