Source code link on lava page needs configuration

Issue Background

At the foot of the LAVA pages there's a source code link which takes you to https://git.linaro.org/gitweb?s=lava%2Flava which isn't a useful location

Issue description

It ought to link to https://git.linaro.org/lava (or maybe https://git.linaro.org/?q=lava- ) this should be done at installation by configuring BRANDING_SOURCE_URL

Acceptance criteria

When the source code link takes the user to a more helpful location