* Add App-level error boundary, referring users to GitHub issue creation on page crashes. (#1452)

* Class component is used as boundary since catching errors is apparently not yet possible with hooks.

* EventListener in window was removed and replaced by error boundary's error catch, which now fires a GA exception. The fields it passes are slightly different because React uses slightly different error types.

* Pre-filling issues with dynamic data is possible with POST requests to GitHub's API, but the GH web client seems to only support pre-fill based on templates. Therefore users still need to copy error info themselves.

* Prefill GitHub issues with crash data.

* Added package 'react-device-detect' to include device data such as OS, browser etc. in crash report.
* Included error stack in issue body.
* Used <code> html tag for displaying stack to user.

* Slightly reduce vertical padding on code block.

* Add ua-parser-js for parsing user agent.

* Revert react-device-detect to ^1.6.2 (which is used for mobile detection etc. in components)