[ads] Custom notification ads

[tmancey]

• [ads] Crash with Custom Notification when set group policy with disabled rewards
• [ads] Crash with Custom Notification when showing a Reminder
• [ads] Icon and close button isn't aligned within the notification
• [ads] Educate users on the mobility and stickiness of custom push notifications
• [ads] Wrong custom notification ad theme when switching between private/normal windows
• [ads] Custom notification ad is attached to a wrong window
• [ads] Implement new custom notification ad design
• [ads] Custom notification ad position was reset to default

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https://github.com/brave/brave-browser/projects/26#column-19651415