Fix proposal form processing to actually use submitted data. Fixes T848
    
    Summary: Changes the way that the bug proposal form is displayed in HTML and processed in Flask so that we're not dropping the submitted data needed to process the blocker request
    
    Test Plan: Load the proposal page, note that the bugzilla username is filled out and the default milestone is properly set. Submit a test blocker/FE (doesn't need to actually go to BZ), make sure that no data was dropped from the form
    
    Reviewers: mkrizek
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1017