[10.3] Prevent login with disabled OAuth providers

See merge request gitlab/gitlabhq!2296

(cherry picked from commit 4936650427ffc88e6ee927aedbb2c724d24b094c)

a0f9d222 Prevents login with disabled OAuth providers

Some providers freeze their name (e.g.
https://github.com/zquestz/omniauth-google-oauth2/blob/414c43ef3ffec37d473321f262e80f1e46dda89f/lib/omniauth/strategies/google_oauth2.rb#L1),
so trying to modify the string would fail with a `can't modify frozen
String` exception (see
https://gitlab.com/gitlab-org/gitlab-ce/issues/37845#note_40308148

).

In this case, we can just stop trying to force the encoding of the
provider name as they should always be in utf8 by default.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Also marks a few things as uncovered, and removes an unused class.

The error saving the user is logged to application.log. Previously,
the entry had no context and was unusable - 'Error saving user:
[Email address already taken]'. Adding the auth hash UID and email
makes the error more helpful.

Fixed privilege escalation issue where manually set external users would be reverted back to internal users if they logged in via OAuth and that provider was not in the `external_providers` list.