Add auto follow back support for instances

client/src/app/shared/users/user-notification.model.ts

config/default.yaml

config/production.yaml.example

package.json

server.ts

server/controllers/api/config.ts

server/controllers/api/server/follows.ts

server/controllers/api/users/my-notifications.ts

server/initializers/config.ts

server/lib/activitypub/follow.ts

server/lib/activitypub/process/process-accept.ts

server/lib/activitypub/process/process-follow.ts

server/lib/activitypub/send/send-follow.ts

server/lib/emailer.ts

server/lib/job-queue/handlers/activitypub-follow.ts

server/lib/job-queue/handlers/video-import.ts

server/lib/notifier.ts

server/lib/user.ts

server/lib/video-blacklist.ts

server/middlewares/validators/user-notifications.ts