chore: support importing resources by fully qualified names (coder#84)

Previously, user, group and template resources could only be imported
via their UUIDs. The `coder/coder` frontend doesn't expose these UUIDs
to the user, so we should provide an alternative, more user-friendly way
to import resources.

User resources: ID or username, since usernames must be unique, even
across multiple orgs.
Template resources: ID or `<organization-name>/<template-name>`
Group resources: ID or `<organization-name/<group-name>`

chore: add missing template data source fields (coder#79)

chore: update readme, add version warnings, examples (coder#71)

Closes coder#69.