Use completed_at datetime field as an indication of a performed migration

closes #7805

Previously we had only a Rails validation which ensured poll participation
uniqueness but this adds uniqueness control to the database level, so that
uniqueness is guaranteed even when changing data with avoiding Rails
validations.

closes #7798

Adds support for merging accounts with account migration by eliminating
duplicate data in favor of the target user's data.

closes #7803

closes #7791

closes #7787

closes #7778

This adds a new dependency (`libidn11-dev` on debian, I'm not sure about
other distros).

This reverts commit 070e4caa.

closes #7772

Disable random tests for now, many tests fail when this is true (which
is now default)