Check announcement banner syntax before saving it, or show a preview

Issue Summary

When adding HTML code to an announcement banner, if there is any syntax issue, the whole JIRA UI will be impacted and users might not be able to login at all if the login form is hidden because of the announcement banner.

Steps to reproduce the issue

• Go to ⚙ > System > Announcement Banner
• Enter the following content in the banner field. Note that the syntax is incorrect, as the closing comment tag is missing a dash character:

  <!-- Commented out code ->
  

• Click on the "Set banner" button
• Log out of Jira and try to log back in
• Notice that the login form is completely gone and that it is impossible to log in:
  

Suggested solution

2 options:

• either add a syntax checker before allowing the admin user to save the banner by clicking on the "Set banner" button
• or provide the admin user with a preview of the announcement banner, so that they can see the impact of it before clicking on the "Set banner" button