feat(dashboards): loosen unique title db constraint on dashboards

[DominikB2014]

This updates the unique title constraint to only apply for non-prebuilt dashboards. This allows us to built prebuilt ones with the same title as an existing user created on. The UI will distinguish the two by the author field

[linear]

BROWSE-142 Update unique title constraint on dashboards model

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1008_loosen_unique_title_contraint.py

for 1008_loosen_unique_title_contraint in sentry

--
-- Alter unique_together for dashboard (0 constraint(s))
--
ALTER TABLE "sentry_dashboard" DROP CONSTRAINT "sentry_dashboard_organization_id_title_6ca343f8_uniq";
--
-- Create constraint sentry_dashboard_organization_title_uniq on model dashboard
--
CREATE UNIQUE INDEX CONCURRENTLY "sentry_dashboard_organization_title_uniq" ON "sentry_dashboard" ("organization_id", "title") WHERE "prebuilt_id" IS NULL;