[6.x] Oath icon fallbacks

[JayGeorge]

This closes #13405

• Adds a Microsoft logo for auth
• Adds screen reader text for accessibility
• Adds fallback text if no icon is present. This will fix the buttons appearing at different sizes.
• Add a tooltip for services
• Makes the icons opaque rather than transparent. I think they were slightly transparency as a hangover from the default way we treat icon buttons. These should probably be opaque otherwise the login services look disabled.

Some screenshots:

Microsoft icon

When no icon is present

Tooltip