GitLab

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

 * 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>

Closes #285.