feat: rename default express session key from express.sid to nodebb.sid

[julianlam]

No description provided.

[nodebb-misty]

💡 Friendly Note

This pull request was made against the develop branch, which is reserved for commits destined for a minor or major release.If your commits simply fixes a bug, please rebase this PR against the master instead.

• patch releases — bug fixes only
• minor releases — new features, enhancements, and bug fixes
• major releases — breaking changes, including all of the above

Thanks!

[julianlam]

Technically not a breaking change, but when merged, would automatically log everyone out unless sessionKey is set in config.json