Add size limit

[azirbel]

Size limit is set to 100KB — need to query the DB, consider what an appropriate limit would be, and email users with active documents over the limit.

This is part of a measure to keep costs manageable. See https://www.npoint.io/changelog for more info, quoted here:

n:point has happily been running for 7 years now, and still works well. I'm proud of it. It's currently handling about 10M requests/day, helping people all over the world run small apps and prototype.
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However, the costs have started to become significant. Since n:point was built to quickly edit small config files, it doesn't cache as effectively as it could, and doesn't have strict size or rate limits. Over time, some large bins that are frequently accessed have contributed to a high bandwidth & storage cost and the site is now costing $500/mo to run.
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I will consider a paid tier, but don't have the time to build it out at the moment. My priority is keeping the site online and free. So I'm updating the site to add usage limits and better caching. Thanks for understanding!

How it works:

• Existing documents are still served as-is. The header is updated with a warning (ignore the 30KB, that's for testing):

• New updates over MAX_CONTENTS_SIZE_BYTES are rejected
• The frontend shows an error as you edit a large document, and saving is disabled