Cache templates, scripts, and content using local storage

[radu-matei]

We currently pull all templates and scripts on each page render, unless the server and browser are configured to allow caching (which, at least for development purposes, for http-server-spa it is not the case, unlike when using http-server).

Before fetching the assets required for rendering a page, we should check the local storage for this, preferably under a revision key from bertrand.yaml.

See https://rustwasm.github.io/wasm-bindgen/api/web_sys/struct.Storage.html.