Add support for custom notification truncate settings

[moraj-turing]

Hello, this PR proposes two basic settings added to a check configuration as a flexible solution to #939

[cuu508]

Thanks for the PR, but I think the right approach here is to make a decision what to truncate, and not make it user-configurable.

Downsides to making it user-configurable:

• this is a niche option, few users will use it. But it will clutter the UI for all users.
• makes product harder to understand. The UI mockup above does not explain when the truncation happens (during ping ingestion, when sending notifications, some other time?)

Also note that the "truncate logs after" limit is dictated by the maximum payload size we can send to various integration channels. Telegram, for example, has 4096 character message limit, other integration types have different limits. Allowing the user to set an arbitrary limit or no limit would result in users sometimes shooting themselves in the foot.

For ping bodies included in notifications, I'm leaning towards truncating the beginning (and keeping the end) like @StevenMassaro in #939 suggested.