Support user management

[florianm]

Feature

Support all API endpoints related to user management:

• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/listing-all-users
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/creating-a-new-user
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/getting-user-details
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/getting-authenticated-user-details
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/modifying-a-user
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/directly-updating-a-user-password
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/initating-a-password-reset
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/users/deleting-a-user
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/app-users/listing-all-app-users
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/app-users/creating-a-new-app-user
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/app-users/deleting-a-app-user
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/roles/listing-all-roles
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/roles/getting-role-details
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/assignments/listing-all-assignments
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/assignments/listing-all-actors-assigned-some-role
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/assignments/assigning-an-actor-to-a-server-wide-role
• https://odkcentral.docs.apiary.io/#reference/accounts-and-users/assignments/stripping-an-role-assignment-from-an-actor

• Figure out a way to test without swamping the test server
• Add vignette(s) for workflows
• Assign to family user-management