Make default max_warnings finite and reasonably low

[Vasfed]

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.

[ashkulz]

@Vasfed I think this is something specific to Gitlab, since #304 implemented it only for GitHub.

I took a look at the gitlab gem, and apparently you can only create a comment one at a time. This seems to reflect the upstream API as well, there's no API for multiple notes like you can do for GitHub.

I'm not sure what's the way forward here, for GitHub having thousands of notes wouldn't be an issue due to the batching. Do you have any suggestions on what the default max_warnings value should be? Happy to accept a PR and have discussion there.