[config.py] Add missing config.source_code_url value

[Olf0]

The string source_code_url only occurs twice:

• config.py, line 50
• about-generator.html, line 13

I.e., there is no other user of this configuration parameter than about-generator.html.
_{I do not comprehend why this link defaulted to the page displayed (https://sailfishos-chum.github.io/about-generator.html), but so what, probably a property of the tooling used.}

Fixes last paragraph of #13 (comment).

[Olf0]

@HenkKalkwater, please review this PR and then PR #15, which goes on top of this PR #14.

I will gladly do the merging, but I first would like your review comprising quality assurance (on technical aspects) and opinion (on all non-technical aspects).

[HenkKalkwater]

The link in the generated HTML is <a href="">…</a> and that apparently simply causes a page reload. This PR looks like how I intended it to work, so thanks a lot!