template: add page_base.html

* Renames page.html to page_base.html and creates new page.html
  to allow overriding of main page template on a Invenio instance.
Merge remote-tracking branch 'jmartinm-github/pu-base-template' into pu
requirements: broken pypi links fix

* Packages removed from pypi have now direct links; this is only a temporary
solution.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
Merge remote-tracking branch 'adrianp-github/pu' into pu
intbitset: usage of separate package