Make base_url available for .hbs to set base href if configured.

[prasad83]

When site_url is configured make it available for hbs to set base href (using base_url)

[prasad83]

Without this change I had to make workaround as follows:

theme/head.hbs

{{#if live_reload_endpoint}}
<!-- local run -->
{{else}}
<base href="https://website.url/">
{{/if}}

[rustbot]

:umbrella: The latest upstream changes (possibly 21f2435182e4a80c0bd0e3184ce36f7a57529d08) made this pull request unmergeable. Please resolve the merge conflicts.