Revert part of #1220

Fixes #1177
The `url` template filter is [only supported in Mkdocs 1.0+](https://github.com/mkdocs/mkdocs/blob/master/docs/about/release-notes.md#internal-refactor-of-pages-files-and-navigation)
Readthedocs.org uses Mkdocs `0.17.3 ` while `make htmldoc` fetches the [latest version from pypi](https://pypi.org/project/mkdocs/) which is `1.0.4`.
Following https://github.com/shaarli/Shaarli/pull/1220, building the docs fails with https://readthedocs.org/projects/shaarli/builds/7886340/