add apple-touch-icon

[lanpa]

When viewing tensorboard with iPad, the server complains:

W0222 03:19:19.907714 Thread-420 application.py:273] path /apple-touch-icon.png not found, sending 404

This patch fixes it.

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[lanpa]

I signed it!

[googlebot]

CLAs look good, thanks!

[chihuahua]

Thank you for the fix. I also filed #994.