Add occupies_seat column to member_roles table

What does this MR do and why?

• This MR follows from discussion, #437719 (comment 1737274716)
• We are trying to improve the query which calculates non-Guest users.
• This calculation gets tricky when users are assigned a custom role, because we have some custom abilities such as read_code which have skip_seat_consumption = true i.e. if a user is assigned to a custom role which only allows read_code as the permission, the user should only be counted as a Guest user. But if the user is assigned to a custom role which allows read_code + read_dependency then they should be counted as a non-Guest user.
• This MR is sub-task 1 of #437719 (closed), it introduces a occupies_seat column in the member_roles table which is set to true if the user is assigned to any one of the non-Guest abilities.

MR acceptance checklist

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Database

Output of db:migrate

Running: `bundle exec rake db:migrate:main`
main: == [advisory_lock_connection] object_id: 107840, pg_backend_pid: 184
main: == 20240206075104 AddOccupiesSeatToMemberRole: migrating ======================
main: -- add_column(:member_roles, :occupies_seat, :boolean, {:default=>false, :null=>false})
main:    -> 0.0032s
main: == 20240206075104 AddOccupiesSeatToMemberRole: migrated (0.0108s) =============
main: == 20240206080928 AddIndexToOccupiesSeatOnMemberRole: migrating ===============
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0048s
main: -- index_exists?(:member_roles, :occupies_seat, {:name=>"index_member_roles_on_occupies_seat", :algorithm=>:concurrently})
main:    -> 0.0037s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0005s
main: -- add_index(:member_roles, :occupies_seat, {:name=>"index_member_roles_on_occupies_seat", :algorithm=>:concurrently})
main:    -> 0.0018s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20240206080928 AddIndexToOccupiesSeatOnMemberRole: migrated (0.0310s) ======
main: == [advisory_lock_connection] object_id: 107840, pg_backend_pid: 184

Output of db:rollback

Running: `bundle exec rake db:migrate:down:main VERSION=20240206080928`
main: == [advisory_lock_connection] object_id: 108600, pg_backend_pid: 124
main: == 20240206080928 AddIndexToOccupiesSeatOnMemberRole: reverting ===============
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0146s
main: -- indexes(:member_roles)
main:    -> 0.0047s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0005s
main: -- remove_index(:member_roles, {:algorithm=>:concurrently, :name=>"index_member_roles_on_occupies_seat"})
main:    -> 0.0015s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20240206080928 AddIndexToOccupiesSeatOnMemberRole: reverted (0.0443s) ======
main: == [advisory_lock_connection] object_id: 108600, pg_backend_pid: 124

Running: `bundle exec rake db:migrate:down:main VERSION=20240206075104`
main: == [advisory_lock_connection] object_id: 108060, pg_backend_pid: 152
main: == 20240206075104 AddOccupiesSeatToMemberRole: reverting ======================
main: -- remove_column(:member_roles, :occupies_seat, :boolean, {:default=>false, :null=>false})
main:    -> 0.0023s
main: == 20240206075104 AddOccupiesSeatToMemberRole: reverted (0.0122s) =============
main: == [advisory_lock_connection] object_id: 108060, pg_backend_pid: 152

How to set up and validate locally

1. In the rails console, create two member roles

# has occupies_seat = false
MemberRole.create(base_access_level: 10, read_code: true) 

# has occupies_seat = true
MemberRole.create(base_access_level: 10, read_vulnerability: true)

Related to #437719 (closed)