Add QA login flow

What does this MR do?

Adds a Flow module for frequently used sequences of actions.

• Adds a login flow, including for logging in, performing some actions and then logging out.
• Updates a test as an example of how to use the login flow.
• Adds a class method QA::Resource::User.admin to provide access to the admin user's details (include username/password needed to sign in)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content