Show 'Enable notifications?' info bar before requesting notification permission

GMail, Discord, Slack, and other websites show a "notification bar" across the top of the web page, asking if the user wants to show notifications, providing a button to "Enable notifications" and a button to dismiss the prompt.

This would make it more obvious as to why Kiwi IRC's requesting permission, and wouldn't require waiting for a mouse click or sending a message.

• Needs new translations
• Needs to handle both mouse and keyboard access to enable or dismiss the notification bar, including e.g. screen readers
• If dismissed, there needs to be a way to re-enable the notification request later
• May need special handling in themes

Continues from the quicker fix in #1206.

Rough draft of issue, feel free to edit!

Example

Ubuntu Discourse's notification prompt bar

Do you want live notifications when people reply to your posts? Enable Notifications. :x: