Make default max_warnings finite and reasonably low

[Vasily Fedoseyev]

When not explicitly set via PRONTO_MAX_WARNINGS or in config, max_warnings is infinite.
This is potentially dangerous, because couple thousands comments can lead to DOS on on-premise gitlab (it's sidekiq becomes flooded with NewNoteWorker/Integrations::ExecuteWorker, also lots of emails are sent and there's a risk of becoming banned by email server for spam), api rate limit hits on other providers etc.

Setting default max_warnings to some reasonable value (150..300..500?) with an ability to explicitly opt-in to infinity seems like a more safe solution.