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