Show a Bootstrap alert in jQuery mode when active content is detected

[Jaifroid]

As discussed in #457, it would be ideal to display a Bootstrap alert when active content that cannot be displayed in jQuery mode is detected. This would be similar to that used in Kiwix JS Windows (see screenshot below), but without the Archive Index feature. @mossroy has suggested the following wording for the alert:

Unable to display active content: this ZIM file is not fully supported in jQuery mode. Try to switch to ServiceWorker mode in the settings (if supported by your platform). Else some content might be available through a search (type some letters in the search zone). [Permanently hide]

It would be ideal if this can be backported in time for release 2.5. The implementation isn't difficult.

[Jaifroid]

PR #467 addresses this issue, and is explained there. I merely add some screenshots here.