Adds proposed changes from #1549.

Summary:
When viewing large graphs in Chrome, clicking the “Download PNG” button
could the error message, “Failed - Network error”. This is because
Chrome does not support downloading large data-URLs. Chrome does,
however, support downloading large object URLs, so this patch makes the
appropriate change to use that API instead.

Test Plan:
Generate the `audio_demo` data and open the `wave:01;sine_wave` graph in
TensorBoard in Chrome. Transitively expand all node groups. Then, click
“Download PNG”. Before this patch, this consistently failed to download.
After this patch, it downloads and image with the correct contents.

Also tested in Firefox.

wchargin-branch: graph-download-png-blob

PR #1512 introduced a bug where it formed a complete path without using
existing pathname. Previously without forming the complete path, it was
treated as a relative path from current window.location.pathname. We decided
to revert back to the old behavior without using `new URL` which was not
providing a clear benefit.

While refactoring the dashboard layout, we inadvertently broke the
horizontal resizing capability. The resizer was clipped and was bound
by the TB global max/min-width limit on the sidebar width.