Allow non-admin users to execute SHOW DATABASES

[mark-rushakoff]

This commit introduces a new interface type, influxql.Authorizer, that
is passed as part of a statement's execution context and determines
whether the context is permitted to access a given database. In the
future, the Authorizer interface may be expanded to other resources
besides databases. In this commit, the Authorizer interface is
specifically used to determine which databases are returned when
executing SHOW DATABASES.

When HTTP authentication is enabled, the existing meta.UserInfo struct
implements Authorizer, meaning admin users can SHOW every database, and
non-admin users can SHOW only databases for which they have read and/or
write permission.

When HTTP authentication is disabled, all databases are visible through
SHOW DATABASES.

This addresses a long-standing issue where Chronograf or Grafana would
be unable to list databases if the logged-in user did not have admin
privileges.

Fixes #4785.

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated
• InfluxData Documentation: [InfluxDB] non-admin users can run SHOW DATABASES docs.influxdata.com-ARCHIVE#1016

[joelegasse]

I think adding an extra type here, then ultimately tying that type back to the user is taking the long road.

I would rather see the UserInfo attached to the execution context, and then just call Authorize for each database instead.

I don't see the benefit in adding a second way to check if a user is authorized for a given database...

[mark-rushakoff]

Per Joe's comment and an offline conversation with him, I'll be pushing up a cleaner implementation most likely today.