Added Visual Review Bot

What does this MR do?

This MR adds visual_review_bot to EE::User#bot_types, along with the associated helper methods and permissions. Most of the code is grabbed directly from the implementation of alert_bot, with some specs added to verify the only permission we want to grant to this robot.

Visual Review Bot will be the comment-authoring entity for Visual Review Tools feedback for #29067 (closed), introduced in !18680 (closed) which can be merged after this.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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

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