style(rubocop): resolve Rails/UniqueValidationWithoutIndex with DB indexes

[mroderick]

Summary

Resolves Rails/UniqueValidationWithoutIndex by adding unique database indexes that back existing uniqueness validations.

Changes

Model	Validation	New Index
AuthService	uid scoped to provider	[uid, provider] unique
Chapter	name unique	name unique
Chapter	email unique	email unique
FeedbackRequest	member_id scoped to workshop	[member_id, workshop_id] unique
FeedbackRequest	token unique	token unique
Invitation	member_id scoped to [event_id, role]	[member_id, event_id, role] unique
MeetingInvitation	member_id scoped to meeting_id	[member_id, meeting_id] unique
WorkshopInvitation	member_id scoped to [workshop_id, role]	[member_id, workshop_id, role] unique
WorkshopSponsor	sponsor_id scoped to workshop_id	[sponsor_id, workshop_id] unique

Also fixed AuthService's validation syntax: constraint: :provider (non-standard, ignored by Rails) → scope: :provider.

Verification

Migrated cleanly against development and test databases with no duplicate-key errors. All model and controller specs pass.

[mroderick]

This PR made me realise that we didn't have any protections against making silly mistakes with adding indexes during migrations, which caused me to create #2774, which added the strong migrations gem.

Now that it has been merged, I've updated this PR to create indexes using disable_ddl_transaction! and algorithm: :concurrently.

It's ready for review!

[olleolleolle]

❤️