The boot script used to be generated in several stages:

 1. The files in `src/boot/` were bundled into `build/scripts/boot.bundle.js`
    with underscore-prefixed placeholders for certain data.

 2. A gulp task would generate `build/manifest.json` which included
    hashes of assets in the build dir

 3. Another gulp task would read `build/scripts/boot.bundle.js` and use
    string replacements to replace the placeholders with final values
    and write the result to `build/boot.js`

This commit simplifies the process by combining stage (1) and (3). Add a
separate Rollup config, rollup-boot.config.mjs, which builds
`build/boot.js` directly from the `src/boot/index.js` entry point, using
@rollup/plugin-replace to replace placeholders with data during the
build.

The code to generate the manifest in step (2) has been replaced with the
manifest-generation code from the @hypothesis/frontend-build package,
which is also shared with other projects.

This change allows removal of several dependencies from the project, and
means that we only have one way to replace `__PLACEHOLDER__`
placeholders in JS files during the build, rather than two different
ones.

One minor change here is that only JS, CSS and sourcemap files are
referenced in the manifest. Font files are not mentioned since the code
that loads them (KaTeX) doesn't make use of the manifest.