Handle implicit issue link at start of line (#2987)
    
    This treats a line starting with #123 - where 123 is a valid
    issue or pull request ID - as an implicit link, rather than as
    a header line.
    
    Doing this is a bit complicated, as markdown does header line
    processing early, in the block processors (that's before the
    inline patterns we usually use). So we can't do this entirely
    with inline patterns. We have to add a preprocessor that runs
    before the block processors, and mungs the relevant strings so
    the block processor doesn't turn them into header HTML. Then
    we adjust the ImplicitIssuePattern to handle these modified
    strings as well.
    
    This also adds tests for implicit links, enabled by mocking out
    enough bits of flask that `_get_ns_repo_user` is happy. There
    are tests not only for implicit links at the start of a line but
    also other cases, including the case of an implicit link not
    surrounded by whitespace that was fixed in the previous commit.
    
    Fixes https://pagure.io/pagure/issue/2987
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>