Fixes CVE-2018-3760

closes #7829

fixes #7770

closes #7774

closes #7814

closes #7822

When public fetch is requested with Poll guid, return parent
StatusMessage for it, which includes the Poll in its turn.

closes #7815

Unescaped asterisks can cause markdown text to be misinterpreted. Also
syntax highlighters (e.g. in sublime text) freak out.

References: [markdown
spec](https://daringfireball.net/projects/markdown/syntax#em)

closes #7821

Closes #7796

closes #7825

Fixes compatibility with newer glibc

closes #7828

See: https://docs.gitlab.com/ee/user/project/import/gemnasium.html

I missed it in #7803

closes #7807

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)