Show "Main contributor" besides "Last contributor" on every page

[segfaultxavi]

It does not feel right to only show the last contributor when sometimes 99% of the commits are from a different person.

Below the "Last updated by" line in the footer we can add something like "Main contributor XYZ with X commits.", also with picture and GitHub link, which shows the author with most commits.

This would need:

• Changing source/_ext/gitstamp.py to retrieve this info from git shortlog -sn and put it in the context variable.
  • Use the git Python package as it is already done, obviously, no more calling external processes and piping output.
  • Rename gitauthor, gitavatar and gitlogin to gitLastAuthor, gitLastAvatar and gitLastLogin.
  • Call the new variables gitMainAuthor, gitMainAvatar and gitMainLogin.
• Changing source/_templates/include/footer.html to render this info.
  • Be careful with the two layouts (desktop and mobile).