Added support for taking screenshot during Notebook publish to Gallery (#108)

* Added support for taking screenshot

- Screenshot is taken using html2canvas package
- Converted to base 64 and uploaded to metadata
- For Using first display output
  - Notebok object is passed instead of string, to publish pane
  - The first cell with output present is parsed out
  - The dom is also parsed to get corresponding div element to take screenshot of the first output

* fixed bug

* Addressed PR comments

- FIxed bug that didn't capture screenshot when mutiple notebook tabs are opened

* removed unnecessary dependencies

* fixed compile issues

* more edits

package-lock.json

@@ -9519,6 +9519,11 @@
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@@ -10874,6 +10879,14 @@
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+    "css-line-break": {
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+      "resolved": "https://registry.npmjs.org/css-line-break/-/css-line-break-1.1.1.tgz",
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@@ -15557,6 +15570,14 @@
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+    "html2canvas": {
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+      "resolved": "https://registry.npmjs.org/html2canvas/-/html2canvas-1.0.0-rc.5.tgz",
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