Jira Connect app UI polish and improvements

What does this MR do?

• Implements design / copy recommendations from #297685 (closed).
  • Smaller font sizes for headings, changed some text to paragraph.
  • Add GitLab UI button style for "Sign in" button.
  • Standardize spacing between sections.
  • Remove "Google Chrome" as recommended browser since it's not really working.
  • Update link when user is signed in to lead to Jira Connect users page instead of to the profile page.
• Fixes a bug where the "Add namespace" button is displayed to non-logged in users. Instead, we should show "Sign in to GitLab". This prevents non-logged in users from seeing the "Add namespace" button and modal and trying to "Link" a namespace when it's not really working.

Screenshots (strongly suggested)

Scenario	Before	After
Without namespaces, not logged in
Without namespaces, logged in
With namespaces, logged in
With namespaces, not logged in

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