[tiny] Introduce GRIST_ACTIVEDOC_TIMEOUT_SECONDS env variable (#2505)

## Context

Currently, there exist no way for an administrator to change the timeout
duration since the last client connection.

## Proposed solution

Introduce the `GRIST_ACTIVEDOC_TIMEOUT_SECONDS` env variable.

This is extracted from this PR: #2336 

## Related issues

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## Has this been tested?

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- [ ] 👍 yes, I added tests to the test suite
- [ ] 💭 no, because this PR is a draft and still needs work
- [x] 🙅 no, because this is not relevant here
- [ ] 🙋 no, because I need help <!-- Detail how we can help you -->