Use unique feature name for Instance Sec Dashboard

What does this MR do?

Use unique feature flag name

This should allow only the Instance Security Dashboard to be disabled, without affecting the other Security Dashboards.

Previously, the Instance Security Dashboard was behind the security_dashboard feature flag. If we disable that flag, we would actually disable the other security dashboards as well, since they are usually guarded by a feature_available?:security_dashboard) check, which implicitly looks for a feature flag of the feature's name, and disables it if and only if the flag is explicitly disabled.

This is to help work around the various issues with the ISD:

• #197369 (closed)
• #196729 (closed)
• #196527 (closed)
• #196526 (closed)
• #198622 (closed)
• #198623 (closed)
• #198625 (closed)
• #198626 (closed)

Migrations

Output for RenameSecurityDashboardFeatureFlagToInstanceSecurityDashboard:

== 20200212014653 RenameSecurityDashboardFeatureFlagToInstanceSecurityDashboard: migrating
== 20200212014653 RenameSecurityDashboardFeatureFlagToInstanceSecurityDashboard: migrated (0.0138s)

Output for RemoveSecurityDashboardFeatureFlag:

== 20200214034836 RemoveSecurityDashboardFeatureFlag: migrating ===============
== 20200214034836 RemoveSecurityDashboardFeatureFlag: migrated (0.0061s) ======

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

/label ~bug devopssecure security dashboard frontend backend