Create empty files so we don't need to nuke tmp/cache in dev

Every startup in development mode, and when plugins are installed or updated in production mode, a blank file is updated, triggering recompilation of the plugin CSS and JS files.

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I have some reservations about this. First of all, the inclusion of a `.plugindata` directory does not sit well in my mind. As was said on meta: https://meta.discourse.org/t/brand-new-plugin-interface/8793/8?u=riking ![mo files mo problems](https://meta.discourse.org/uploads/meta_discourse/1519/ee4aeb2fdbb33509.jpg)

Also, is the line to add that directory, which I'm unsure about, to `Rails.configuration.assets.paths` in the right place? It can't go in `Discourse.activate_plugins!` because otherwise Rails will complain when you try to `require_asset` a path that isn't in the asset path (and rightly so!) when the plugins aren't loaded.

From a security standpoint, having this hidden directory is mostly okay - `depend_on` won't actually put the files in, so you can't use them to hide some evil JS from the administrator (and that stuff would be nuked on a rebootstrap anyways).