Swap 'access_level' for 'role' in access token entity

What does this MR do and why?

These new serializable group and project entities are going to be used in conjunction with a Vue component. This frontend component consumes role the human_access (string) version of the access_level (integer).

I have asked if there was a JavaScript function to convert from the integer version to the string version, and I was recommended to make the change in the backend instead: https://gitlab.slack.com/archives/C0GQHHPGW/p1661259205181069

The group and project entities are not yet in use.

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