feat: phx_web generate endpoint.ex with CheckRepoStatus plug migration_lock: false option

[cgrothaus]

Goal

phx_web generator: generate CheckRepoStatus plug line with an option so that it runs without migration lock.

Background

phoenixframework/phoenix_ecto#160 makes it possible to pass a migration_lock option to CheckRepoStatus plug. This plug is a convenience that checks for pending migrations. By default it uses the migration lock strategy defined in the OTP app ecto repo configuration. When that is configured to :pg_advisory_lock, concurrent requests in dev environment may be delayed by 5s. See this elixir forum topic for more information.

IMO just reading the pending migrations should be possible without any migration lock. Hence this PR adds the migration_lock: false config option to CheckRepoStatus plug.

Dependencies

This PR depends on the changes in phoenixframework/phoenix_ecto#160, so first phoenix_ecto has to be released.

[chrismccord]

Given this is a dev feature I don't think we need specialized behavior and the current checks are preferred since they are more comprehensive? Thanks!