Add transition for new-tabbackground

[cezaraugusto]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Ran git rebase -i to squash commits (if needed).

Test Plan:

to avoid flash of unstyled content

Auditors: @bsclifton

Closes #5309

Test Plan:

• Enable new tab page
• Open a new tab
• There should be a smooth transition between the moment page is opened and background is charged

[bsclifton]

Looks great! Will merge here in a moment... @bradleyrichter: can you please get latest and try this out? Since the pictures need to load remotely, there was a noticeable delay which ended up causing a flash. This resolves that by gradually fading in (over 200ms)