[Regression] Can't add retention policy to a channel if there is no global policy for it

[vynmera]

In one of @ggazzo 's commits, the first removed OverrideGlobal switch was readded. This is fine, except when you need to set a per-channel policy in a server without a global policy.
The OverrideGlobal switch only appears when a global policy is set. This would make sense (there is no global policy to override, after all), but it poses an issue.
All subsequent switches (e.g. max age, exclude pinned etc.) require the OverrideGlobal switch to be set. Since there is no global policy, we can't flip that switch, and as such are locked out of interacting with the policy at all.
Turns out it was mostly just due to erroneous templating. Fix is in!

Fix here: #11927