LDAP email address downcasing

Fixes #2960

In the event we cannot match an LDAP user by DN we attempt to find an identity by email address and then update the DN. In this case the identity is matched by email address. 

If the user's email address in LDAP has an upper case character we cannot find a match in the GitLab database. GitLab downcases emails before the user object is saved.

This merge request downcases the email from LDAP before we lookup by email. I also added a test to prevent a regression.

See merge request !1550

Normalize space-like characters in keys before output to gitlab-shell

gitlab-shell expects only one tab separator per key, and an SSH key with
a tab character in the comment, for example, would break things.

Closes #2970

See merge request !1552

Add task to disable 2FA for all users

Addresses #2971 

See merge request !1532

gitlab-shell expects only one tab separator per key, and an SSH key with
a tab character in the comment, for example, would break things.

Closes #2970

This ensures any PostgreSQL specific schema changes (e.g. expression
indexes) are created when setting up the database.

These two methods remove the need for manually going into
ActiveRecord::Base.connection all over the place.

See merge request !1512