Remove border around new tab button; also increases width of button

[bsclifton]

Remove border around new tab button; also increases width of button making it much easier to click)

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fixes #6467
Also fixes #6125

Regression was caused by the combination of these two commits:
010e708
615a6ec

Auditors: @bbondy, @bradleyrichter

How it looks with these changes

Hitbox is now 25 wide by 25 tall, instead of 15 wide by 25 tall

mini-video of it in action

How it looked before these changes

comparison to the old one (crowd boos):

and for good measure, here's the border issue that this fixes (which I could only repro on Windows at > 100% DPI):

[bsclifton]

@bradleyrichter added!

[bradleyrichter]

I reviewed on Mac. Looks good. Nice large hit area while retaining previous icon size.