Add customizable user roles

Default user roles are:

"Moderator"
"Admin"
"Owner"

Example role ideas:

"Analyst", with only access to "View Dashboard", allowing retrieval of various metrics
"Auditor", with only access to "View Audit Log", allowing the perusal of the log of administrative actions
"Content reviewer", with only access to "Manage Taxonomies", allowing the review of trending content

https://github.com/mastodon/mastodon/pull/18641