Classic CSS tweaks for Thunderbird Quantum v1.0.2

[GENERAL]

• added @imports for my_userChrome.css and my_userContent.css files

[Buttons]

• (new) hide panel arrow from button popups
• (tweak) classic button appearance for main toolbars > support for Lightning notification bar

[General UI]

• (new) increase row height
• (tweak) attachment box above mail content