84 add version control url

[RMcNeely]

Resolves #84

Adds support for providing a link to the source code on individual app pages.

• New parsing option for vcs-browser when parsing Appstream data.
• New button at the footer on individual pages to link to source code

[RMcNeely]

Somehow my fork got really messed up by the GHA automatic update. I'm not sure why but now I'm 100+ commits ahead of the elementary repo :(

[ghost]

Hi! Is there a reason this is still open? Is there something that needs to be fixed?