refactor(web): common template for index pages of tvl and atward

Use simple string interpolation based approach to templating and allow
changing the main body, the title and to inject extra HTML into the head
element. Additionally we can use `https://tvl.fyi/` instead of `/` when
referring to assets.

One limitation currently is that the template only works for index pages
(it link to self using `href="/"`), but this should be easy to fix.

For atward, instead of using the `onload` attribute of `body`, we now
register an event listener in JavaScript which makes the template code
less complicated. When building the derivation the template is rendered
to HTML and injected into the source.

Change-Id: I2ea0c5bf5f6286e781285ade7751a348bab3bdc8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3112
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/atward/build.rs

@@ -0,0 +1,50 @@
+//! Build script that can be used outside of Nix builds to inject the
+//! ATWARD_INDEX_HTML variable when building in development mode.
+//!
+//! Note that this script assumes that atward is in a checkout of the
+//! TVL depot.
+
+use std::process::Command;
+
+static ATWARD_INDEX_HTML: &str = "ATWARD_INDEX_HTML";
+static ERROR_MESSAGE: &str = r#"Failed to build index page.
+
+When building during development, atward expects to be in a checkout
+of the TVL depot. This is required to automatically build the index
+page that is needed at compile time.
+
+As atward can not automatically detect the location of the page,
+you must set the `ATWARD_INDEX_HTML` environment variable to the
+right path.
+
+The expected page is build using the files in //web/atward/indexHtml
+in the depot."#;
+
+fn main() {
+    // Do nothing if the variable is already set (e.g. via Nix)
+    if let Ok(_) = std::env::var(ATWARD_INDEX_HTML) {
+        return;
+    }
+
+    // Otherwise ask Nix to build it and inject the result.
+    let output = Command::new("nix-build")
+        .arg("-A").arg("web.atward.indexHtml")
+        // ... assuming atward is at //web/atward ...
+        .arg("../..")
+        .output()
+        .expect(ERROR_MESSAGE);
+
+    if !output.status.success() {
+        eprintln!("{}\nNix output: {}", ERROR_MESSAGE, String::from_utf8_lossy(&output.stderr));
+        return;
+    }
+
+    let out_path = String::from_utf8(output.stdout)
+        .expect("Nix returned invalid output after building index page");
+
+    // Return an instruction to Cargo that will set the environment
+    // variable during rustc calls.
+    //
+    // https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-envvarvalue
+    println!("cargo:rustc-env={}={}", ATWARD_INDEX_HTML, out_path.trim());
+}