Add validators for the Dissemin papers and website

Validation has been added for two types for variables noticed in the
translations - %s / %d and %(names)s / %(count)d.

Github issue - https://github.com/dissemin/dissemin/issues/792

Plural validations will be added as part of a separate patch.

Bug: T224025
Change-Id: I7091873dc05c356bb6a526feb300097d05346069