pagure_bot: handle "FE" abbreviation in a safer way
    
    Instead of considering both variants everywhere in our code, always transform
    "fe" into "freezeexception" right after parsing it. This way we can only consider
    a single version of each tracker name internally in most of our code.
    
    Related: https://pagure.io/fedora-qa/blockerbugs/pull-request/163