Feature Request - New Update Available Alert in Admin dashboard

[Cave-Johnson]

Describe the feature you'd like
An indicator next to the version in the bookstack admin panel stating that a new version is available. Similar to Gitlabs "Update Available / Update ASAP"

Describe the benefits this feature would bring to BookStack users
This would allow bookstack admins to easily see that a new version was available and if a new release is out that contains critical security fixes, would help admins know that there is an update out.

Additional context
This would help admins not signed up to newsletters / email updates and who don't check git regularly

[siegerhansma]

Took a first stab at it in #2859