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>