Remove / Disable analyitics in self-hosted versions

[jamesread]

Postiz currently uses Plausible analytics and PostHog in every page load (see apps/frontend/src/app/layout.tsx), which is likely to be undesirable for most self-hosted users who are quite privacy concious, and whow would not be able to see data sent from their Postiz installations.

Could we add ENABLE_PLAUSIBLE and ENABLE_POSTHOG, which if set to "true", would then load the Plausible/PostHog elements? (ie, default to not using the elements if that variable is unset?).