Make photo shown in language menu configurable

This change aims to make the picture (and the hyperlink) for the photo
shown in the language menu configurable. That way, future versions of
the bot should not require additional patches when a rebrand occurs or
necessarily depend on external, cosmetic web resources to function.

The bot should still work if the user forgets to upload the .yaml file,
but, more importantly, it should also work if the hyperlink we use ever
goes down. Still, please make sure to update your .yaml file.

"Which hyperlink should I use?" is a question that I asked myself when
I included it in the first place. I found that using a permalink of a
commit on GitLab was a better idea than using the hyperlink to the
image on the website itself. I made the following assumptions to reach
that conclusion:

- The website is going to be changed and updated more often than the
  software used to host source code repositories.
- Nobody will think about this project before updating the website.
  And understandably so. We should act that way.
- I do not expect the picture in the repository to change, but I expect
  it to be replaced or moved someday.
- Using the Internet Archive to store a .png would be a bit overkill.