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

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

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

By using a JOIN we can remove the need for using 2 separate queries to
find a project by its namespace. Combined with an index (only needed for
PostgreSQL) this reduces the query time from ~245 ms (~520 ms for the
first call) down to roughly 10 ms (~15 ms for the first call).

phantomjs < 2.0 doesn't support creating `KeyboardEvent` so the tests
were failing on CI

On PostgreSQL these queries use LOWER(...) to compare columns and
values. For MySQL a regular WHERE is performed as MySQL is already
case-insensitive.

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