Avoid exposing unaccessible repo data upon GFM processing

When post-processing relative links to absolute links
RelativeLinkFilter didn't take into consideration that
internal repository data could be exposed for users
that do not have repository access to the project.

This commit solves that by checking whether the user
can `download_code` at this repository, avoiding any
processing of this filter if the user can't.

Additionally, if we're processing for a group (
no project was given), we check if the user can
read it in order to expand the href as an extra.
That doesn't seem necessarily a breach now,
but an extra check doesn't hurt as after all
the user needs to be able to `read_group`.