Separate SCIM Identities from SAML

What does this MR do?

NOTE: Contains feature flag - :scim_identities scoped to Group

This MR is the outcome of the decision in #207553 (closed). Implementation issue: #208940 (closed) (does not fully close)

This lays the framework for better SCIM support in the future. SCIM identities will now be separate from SAML identities.

When the feature flag is disabled the current behavior is preserved. When enabled, these changes only include the successful lookup of a SCIM identity via the ScimFinder class. Other changes necessary to add new identities or get specific identities within the SCIM API will come later.

I did not add a CHANGELOG entry because this isn't feature complete and is behind a feature flag. I will add an entry to the final MR when the feature is released.

Database Migration Output

== 20200227164113 CreateScimIdentities: migrating =============================
-- create_table(:scim_identities)
   -> 0.0516s
== 20200227164113 CreateScimIdentities: migrated (0.0517s) ====================

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team