frontend: web-ui: server-side pagination for too-many-builds
    
    When the list of builds is larger than N (10.000 currently), instead of
    loading all-in-one page and sort by JavaScript - provide a server-side
    pagination feature.
    
    This required a new `current_url()` helper (in templates as well).
    
    Note that I had to add 'dataTable' class (along the already existing
    'datatable') to drop the 'margin-bottom: 20px' value causing an ugly gap
    between pagination and the table.
    
    We now also import all context_processors.py (not only the given list)
    into coprs.__init__.py.
    
    Fixes: #54