Add occ commands to pregenerate CSS and JS files

[rullzer]

Just so it doesn't have to happen on demand.

As an added bonus we could add a '--rebuild' flag or something. So that if people somehow mess up their files in the appdata dir. That we just regenerate everything.

@skjnldsv does this make sense to you?

[skjnldsv]

So we could have a fix to give to users who have a broken setup?

[juliusknorr]

Makes sense to have, basically this is what is left in #6587.

[juliusknorr]

We should also fore rebuild during upgrade when implementing this.

[juliusknorr]

PR to clear the caches during update / occ maintenance:repair is in #7984

[MorrisJobke]

@juliushaertl Something for the polishing phase or should we just drop this in favor of the idea to not generate it during runtime, but during build time?