Allow sign-in via Atlassian Cloud

What does this MR do?

Allows users to sign-in via Atlassian cloud.

• Allows new users and existing users with a linked identity to sign-in via the button on the sign-in page.
• Allows existing users without a linked identity to link it via the accounts page.

This is accomplished via the new Atlassian Identity model and associated database table added via !40176 (merged), which will allow GitLab users to connect their account to Atlassian Cloud. GitLab will store the OAuth token and refresh token so GitLab can interact with Atlassian JIRA via the API. This will enable further integrations beyond authentication in the future.

This MR is larger than I preferred, but I didn't see a logical place to separate it. Most of the length is due to tests.

Related to #220991 (closed)

Screenshots

When Atlassian account not yet connected/linked

When Atlassian account connected/linked

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