Large pdf improvements

[hoxbro]

Fixes #3696

The method used to be able to render large PDFs is the one described here. Which converts the base64 to a blob and inserted as an URL with URL.createObjectURL(blob); in the Typescript Bokeh model.

I have also added start_page to be able to set the start page when loading the PDF.

The pane is located in image.py and not in markup.py because it is a subclass of FileBase, which is located in Image.py.

Todo

• [ ] Confirm that this is the method to go forward with.
• [x] Write unit tests for this method.
• [x] Update existing unit tests to the method.
• [ ] Maybe add other URL parameters navpanes=0&scrollbar=0&toolbar=0&zoom=100?
• [ ] If there is some way to cache the data of convert_base64_to_blob.

[codecov[bot]]

Codecov Report

Merging #3716 (f1fab92) into master (ff39f02) will increase coverage by 0.66%. The diff coverage is 95.74%.

@@            Coverage Diff             @@
##           master    #3716      +/-   ##
==========================================
+ Coverage   83.56%   84.22%   +0.66%     
==========================================
  Files         207      214       +7     
  Lines       29654    31033    +1379     
==========================================
+ Hits        24779    26137    +1358     
- Misses       4875     4896      +21     

Flag	Coverage Δ
ui-tests	34.09% <85.10%> (+2.18%)	:arrow_up:
unitexamples-tests	77.05% <46.80%> (-1.14%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/tests/pane/test_image.py	100.00% <ø> (ø)
panel/pane/image.py	80.28% <83.33%> (-1.50%)	:arrow_down:
panel/models/__init__.py	100.00% <100.00%> (ø)
panel/models/markup.py	100.00% <100.00%> (ø)
panel/tests/ui/widgets/test_image.py	100.00% <100.00%> (ø)
panel/tests/widgets/test_slider.py	98.89% <0.00%> (-1.11%)	:arrow_down:
panel/config.py	59.55% <0.00%> (-0.45%)	:arrow_down:
panel/tests/test_param.py	99.67% <0.00%> (-0.21%)	:arrow_down:
panel/tests/test_depends.py	100.00% <0.00%> (ø)
... and 21 more

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