Closes #1275

Closes #1268

Closes #1267

Closes #1263, #431

Add the sidetiq gem to the Gemfile to match EE.

Remove emphasis from system notes to avoid Markdown conflicts in names.

Changes:
- change `.navbar-toggle` from `margin: 0 -15px 0 0;` to `margin: 0;`

Use more restrictive permissions for backup tar files and for the db,
uploads, and repositories directories inside the tar files.

This reverts commit c42262b4, reversing
changes made to c6586b12.

Fixes issues #880, #723, #1113: Markdown must be rendered to XHTML, not HTML, when generating summary content for Atom feeds. Otherwise, content-less tags like <img> and <hr>, generated when issue descriptions, merge request descriptions, comments, or commit messages use emoji, horizontal rules, or images, are not terminated and make the Atom XML invalid.