zoekt-webserver: Support non-root hosting

[dharesign]

zoekt-webserver seems to assume that it's being served at the root of a domain. This is true for basic use cases, but not if it's put behind a proxy that maps e.g. domain.com/a/ and domain.com/b/ to different instances.

Example issues:
https://github.com/sourcegraph/zoekt/blob/main/web/templates.go#L101
https://github.com/sourcegraph/zoekt/blob/main/web/templates.go#L309-L313

It would be great to have a command line option for zoekt-webserver like -web_root /a, and then have that plugged through to the templates so everything could be:

<a class="navbar-brand" href="{{.WebRoot}}">Zoekt</a>
<dt><a href="{{.WebRoot}}/search?q=needle">needle</a></dt><dd>search for "needle"</dd>

[linear]

CU-2274

[keegancsmith]

Yes in practice I think this would actually be relatively straightforward to support.

Alternatively, could zoekt use relative URLs instead. eg instead of /search?q=needle I think doing ./search?q=needle might work? But I am just spouting that without looking at any of the code to check.

FYI very happy to code review and test any PRs here :)