Update pdfjs to v2.9

[AlexAndBear]

Description

• Update pdfjs from v2.5 to v2.9 (which enabled signature support)
• Drop support for IE and non-chromium-based EDGE due to pdfjs cancel support since v2.7
  • Download will be started instead of opening with files_pdfviewer while click on a pdf file
  • Context menu item 'Open in PDFViewer' won't be displayed anymore

Fixes

#286
#258
https://github.com/owncloud/enterprise/issues/4680

Might fix:
#275

Screenshots

Before:

After:

[CLAassistant]

All committers have signed the CLA.

[AlexAndBear]

Ready to review

[jvillafanez]

I'm not sure if falling back to download the file is a good idea. It might be better to show a "your browser isn't supported" notification. The problem I see is that the behaviour is very different: I'm on my phone / tablet, touch the file and it opens the pdf, however when I'm on the office using IE 11 (company policy) and I click the file, it downloads instead of opening. It seems like a bug to me.
At least, with the notification, the user knows what happens and can choose between downloading the file or use a different browser to open the file.

[AlexAndBear]

@jvillafanez this has been discussed with @hodyroff and @tbsbdr as files_mediaviewer does the same, the decision was made.
I brought in the same objection as you in the first place ;) I think I might show a console.warning

[jvillafanez]

I still think there will be bug reports incoming complaining about the behaviour, but if it was decided... 🤷‍♂️

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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