When this code was moved from being in zerver in 21a2fd482e, it kept
the `if ZILENCER_ENABLED` blocks. Since ZILENCER and CORPORATE are
generally either both on or both off, the if statement became
mostly-unnecessary.
However, because tests cannot easily remove elements from
INSTALLED_APPS and re-determine URL resolution, we switch to checking
`if CORPORATE_ENABLED` as a guard, and leave these in-place.
The other side effect of this is that with e54ded49c4, most Zulip
deployments started to 404 requests for `/apps` instead of redirecting
them to `https://zulip.com/apps/` since they no longer had any path
configured for `/apps`. Unfortunately, this URL is in widespread use
in the app (e.g. in links from the Welcome Bot), so we should ensure
that it does successfully redirect.
Add the `/apps` path to `zerver`, but only if not CORPORATE_ENABLED,
so the URLs do not overlap.
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