Change looks of thread and message views.

• The grey background for the email headers are removed.
• Gravatars are now round in shape instead of square.
• Links to permalink of the message and fixed-font button has been moved to the bottom to remove clutter from the top of the message.
• The date format is changed to remove the Day of the week and uses 3 letter month names for minimalism.
• Right sidebar is now fixed and doesn't scroll with the page.
• All buttons on the bottom of the email now have a hover property that changes their background on mouse hover.
• Quoted part of the email now has a different background. Each level of quoting gets a different background till 3-levels of indentation. After, which we don't process more indented text.
• Unread threads show a blue bar on the left side as a more prominent indication as compared to current envelope icon.