Refactor and redesign chat page, empty chat page and chat message bubbles

• Create base for empty and (normal) chat page
• Replace background image by background image with transparent background to adjust background color for light and dark themes
• Decrease spacing between chat message bubbles
• Use less rounded corners for chat message bubble
• Use glow effect around chat message bubble instead of drop shadow
• Use consistent text color for own messages and messages of chat partners
• Use background color as chat message bubble color for own messages
• Use mix of highlight color and background color as chat message bubble color for messages of chat partners
• Use same style for chat page and message bubble as for empty chat page and message bubble
• Improve chat message bubble proportions

Fixes #335, #338.

Signed-off-by: Linus Jahn <lnj@kaidan.im>