Gitlab from sources : proper utf8mb4 support for MySQL 5.5 to 5.7

What does this MR do?

• It reverts the DB encoding from utf8mb4 to utf8 introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7420

• The GitLab installation process will fail if utf8mb4 is used with a MySQL < 5.7.7 instance https://gitlab.com/gitlab-org/gitlab-ce/issues/26268

• Future GitLab DB migrations creating tables using large indexes or adding such indexes on existing tables will fail with a MySQL < 5.7.9 instance: https://gitlab.com/gitlab-org/gitlab-ce/issues/26268#note_21173979

• It documents how to properly support and enable utf8mb4 with GitLab using MySQL 5.5 to 5.7.

• It document a proper maintenance for a consistent GitLab DB, free from encoding/collation mistmatch as it is the case now (new tables/columns will be created with the wrong encoding/collation expected by the MySQL2 adapter connection).

Why was this MR needed?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/26268#note_21113895
• https://gitlab.com/gitlab-org/gitlab-ce/issues/26268#note_21173979