Fix admin mode when authenticating with LDAP

What does this MR do?

Fixes #331108 (closed)

The LDAP spec says that DNs are case-insensitive and that whitespace doesn't matter. Sometimes LDAP servers do weird things and return DNs in differing cases or with differing whitespace. In GitLab we normalize DNs before storing and should similarly always do case-insentive, normalized lookups.

This changes the User#matches_identity? method to use the existing scope which does these things.

Rails console examples:

Before:

user.matches_identity?('ldapmain','uid=john,ou=people,dc=example,dc=com')
=> true
user.matches_identity?('ldapmain','uid=john,ou=people,dc=example, dc=com')
=> false
user.matches_identity?('ldapmain','uid=john,ou=people,dc=example,DC=com')
=> false

After:

user.matches_identity?('ldapmain','uid=john,ou=people,dc=example,dc=com')
=> true
user.matches_identity?('ldapmain','uid=john,ou=people,dc=example, dc=com')
=> true
user.matches_identity?('ldapmain','uid=john,ou=people,dc=example,DC=com')
=> true

SQL Query:

Before:

SELECT
    1 AS one
FROM
    "identities"
WHERE
    "identities"."user_id" = 104
    AND "identities"."provider" = 'ldapmain'
    AND "identities"."extern_uid" = 'uid=john,ou=people,dc=example, dc=com'
LIMIT 1

After:

SELECT
    1 AS one
FROM
    "identities"
WHERE
    "identities"."user_id" = 104
    AND (LOWER("identities"."extern_uid") = LOWER('uid=john,ou=people,dc=example,dc=com'))
    AND "identities"."provider" = 'ldapmain'
LIMIT 1

Screenshots (strongly suggested)

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