WIP: #190 JSON export

[GneyHabub]

Now there are 2 buttons over each Content field of the Messages table.

If you hover over any of them there will be a popup hint which tells the user what this button does:

The first button will copy the Content to the clipboard and the second will trigger a file download (.json if the content is an object or JSON.parse doesn't throw an exception; .txt otherwise.)

[workshur]

1. Please add description
2. Add screenshot

[workshur]

1. We need to move buttons to JSONViewer component.
2. Use standard bulma buttons

[GneyHabub]

I have no idea how to test the file download yet. Still working on it.

[GneyHabub]

Okay, after some research I've got an impression that the file-downloading part of the functionality is pretty much untestable. I didn't find a way to trigger the file download inside of the testing environment.
It is possible to dig into the specific implementation and test it, but this seems wrong to me. In this way, we treat the component as a white box, exposing all of its guts, tying our tests to the specific implementation. Looks like a violation of a bunch of principles and good practises.
@workshur @whotake what do you think?

[whotake]

@GneyHabub Does it throw any errors when are you trying to test it?

I think we can split view and download logic and test them separately:

1. Test view that it calls saveFile with proper arguments
2. Test saveFile itself. And there a few options on how to do it:
   2.1 Split function into modules and test important logic. We care about file name and content more than pretty much standard downloading function.
   2.2 Mock document and check setAttribute agruments
   2.3 Combine two methods above

[sonarcloud]

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