[kiwix-serve] make the topbar customizable.

[mgautierfr]

At BSF, we need to change how the topbar is displayed. (Css but also displayed buttons).

I've got a working version, but it is based on some kind of hidden feature of the last kiwix-serve/kiwix-lib allowing to change the embedded resources at runtime with env var.
It's working but it is not perfect and error prone. (ideascube/ideascube#590 and ideascube/ansiblecube#43)

On top of it, we will end to rewrite a part of the topbar to make it more mobile friendly. (kiwix/kiwix-xulrunner#336)
It could be a good opportunity to think about how we add the topbar.

[kelson42]

@mgautierfr We have meanwhile rewritten the topbar in a more friendly manner. I wonder a bit how such a feature would/could looks like, considering that it should probably deal with HTML/CSS/JS/pictures.

[kelson42]

@mgautierfr I get once kiwix/libkiwix#394 is implemented, we could just use a similar approach like kiwix/libkiwix#571 ?!

[kelson42]

Moving this task to @veloman-yunkan, this is IMO now straigt and trivial to implement this option, once kiwix/libkiwix#716 will be merged.

[kelson42]

We should not forget to update the README.md