[Notification] Rework APIs for better modularization (#48876)

src/metabase/channel/core.clj

src/metabase/channel/impl/email.clj

src/metabase/channel/impl/http.clj

src/metabase/channel/impl/slack.clj

src/metabase/core.clj

src/metabase/db/custom_migrations.clj

src/metabase/email/alert_new_confirmation.mustache

src/metabase/email/new_user_invite.mustache

src/metabase/email/slack_token_error.mustache

src/metabase/events.clj

src/metabase/events/notification.clj

src/metabase/models/notification.clj

src/metabase/notification/core.clj

src/metabase/notification/payload/core.clj

src/metabase/notification/payload/impl/system_event.clj

src/metabase/notification/send.clj

src/metabase/pulse/send.clj

src/metabase/util.cljc

src/metabase/util/namespaces.clj

test/metabase/api/channel_test.clj