Clear cache correctly upon extension enable/disable/update

[clarkwinkelmann]

Until flarum/framework#837 (comment) is fixed in core maybe we should clear the manifest.json ourselves so assets get a new name and bypass the old version from Cloudflare/Proxy ?

Might also help against the CTRL+F5 CTRL+F5 CTRL+F5 CTRL+F5 CTRL+F5 effect

[luceos]

I've created a new label to keep track of stuff we might do to patch core. If we want to do this, we should fix it in Bazaar and tag it asap, but also PR against core.