Add GitHub repository to sidebar

[hobofan]

CLOSES #117

Adjustments to metadata/Github repo link:

• Adds a link to the GitHub repository to sidebar
• The label for the hompage link is now just "Homepage" (better consistency across pages)
• Add icons for both repo and homepage links
• If the homepage and repository links are redundant (very often the case, as the homepage field is required metdata and often set to the repository page), the homepage link is omitted, and only the repo link is displayed

Also includes some smaller styling adjustments/visual bugs:

• For phone screen sizes the metadata section is moved below the other content (not as a sidebar anymore)
• Reduced the top margin of the metadata sidebar so that it is consistent with the other content
• Better wrapping for "long" maintainer names
• Sidebar is less squished for modules with short name (e.g. rules_go)

Example screenshots

Old (rules_aspect_js):

New (rules_aspect_js):

Old (rules_go):

New (rules_go):