MOTM-289: Attaching polls from second page of polls list is not possible

Add missing article nr param to pagination links.
MOTM-301: Registration error on username conflict

Add form error on username conflict. Remove break.
Revert "MOTM-289: Attaching polls from second page of polls list is not possible"

This reverts commit 55c42f3f51edd22d3381a7d13dd19b260161eba9.
Merge branch 'fix-registration-41' of https://github.com/petrjasek/Newscoop into v4.1
Merge branch 'fix-debate-assign-41' of https://github.com/petrjasek/Newscoop into v4.1