feature/featuretags: add a catch-all "Debug" feature flag

Saves 168 KB.

Updates #12614

Change-Id: Iaab3ae3efc6ddc7da39629ef13e5ec44976952ba
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f