Severity statistics and csv export

[csordasmarton]

• Showing severity report count at the statistics page.
• Statistics can be exported to CSV.

Closes #1104

[whisperity]

The button feels weird for me. It could be that the alignment of the text is off a few pixels, or the colour is too saturated.

[csordasmarton]

@whisperity You have good eyes to catch pixel alignment problems 😄 I have fixed this and I have also changed the color of the export button:

What do you think?

[whisperity]

Looking good now. I think we could omit the : here.

[gyorb]

Do we have the same functionality at the command line? I prefer to add the export features there first for easier automation. With the UI buttons users have to do the result download manually if they need. In some cases this can be required, but I still prefer the solutions where automation is possible.

[whisperity]

I think this feature (checker stats) is CodeChecker cmd sum which does have an --output <format> option.

[csordasmarton]

We have a --output <format> option at CodeChecker cmd sum. For more information check the user guide: https://github.com/Ericsson/codechecker/blob/master/docs/user_guide.md#cmd-sum

[whisperity]

I think this is ready to go in the next release after the current one.