Create a new members finders

A group or project members are currently retrieved using a combination of:

• Project#project_members
• Project#users (which is a has_many through #project_members), usually used when we need to search, using User.search
• Group#group_members
• ProjectTeam#members and its #users alias (this one is really confusing)

We should create a new either a single MembersFinder class or two ProjectMembersFinder & GroupMembersFinder classes that would abstract away all the calls to model methods and scopes. /cc @rspeicher