Instance level Security Dashboard as an option for default dashboard

Problem to solve

With the introduction of the Instance level Security Dashboard we want to make GitLab focused also on security teams.

These users will be more interested in the Security Dashboard first, so we should include the ability to make it the default when they log into the UI.

Proposal

Add the ability to select Security Dashboard in User Settings > Preferences > Behavior > Default dashboard.

BLOCKED BY: https://gitlab.com/gitlab-org/gitlab-ee/issues/6386

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