UX improvements for backpacks

In MDL-63262 has been added support for more than one backpack. While reviewing the issue, I've seen some improvements could be done to let users configure and use backpacks easily:

• The "Add to backpack" button shouldn't be displayed if the backpack is not configured properly. For instance, when the Badgr.io backpack isn't configured with the correct issuer's password and a user tries to send a badge, the following error is displayed: "Invalid request".
• The "Backpack settings" and the "Manage backpacks" administration pages could be merged, as it was done with the "Notification settings" in MDL-64495.
• The warning message added in MDL-65518 should be reviewed to make it more consistent with similar testing messages (such as the 'Testing settings' column in 'Site administration / Plugins / Authentication / Manage authentication'). Look at the screenshot backpack_warning.png