[fix #124] Make default API prefix relative

When using \BlueLaTeX from behind a proxy, if the API prefix is
absolute, then it might lead to some problems because the client send
the request to `/api`.
This is a problem iF \BlueLaTeX is not accessed at the root before
the proxy rewrites the URL.
Merge branch 'satabin-fix/wrong-default-api-path'
Fix file change

The content of the page is not anymore deleted when the user change the
file.