codimd: js-sequence-diagrams: use git source rather than the NPM tarball

It seems as NPM just removed the tarballs of the unpublished package,
hence `codimd` isn't buildable. The sources for the package are
available on github[1] and fix the build.

For further information about the `js-sequence-diagrams` workarounds,
please refer to 5feec424.

[1] https://github.com/Moeditor/js-sequence-diagrams