Adds rounded borders to the top corners for Windows 7

[bsclifton]

• 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).

Adds rounded borders to the top corners for Windows 7

• Introduces a platformUtil helper
• Top level element uses this helper to add classes for platform and release (Windows only)
• removed OS detection logic in WindowCaptionButtons component
• reworked styles in navigationBar.less

Manually tested on:

• Windows 7
• Windows 10
• Windows 7 at 150% DPI (corners didn't look exactly right, but might be a VM issue)

Fixes #4216

Auditors: @bbondy @bridiver

[bsclifton]

There are two methods in platformUtil which are not used: isDarwin and isWindows. I was thinking maybe these could be used in place of all the const isDarwin = process.platform === 'darwin' calls we have (but felt it was out of scope for this change)

[bbondy]

++