Add optional diagnostic info into "export to json"

[expln]

There should be an option to include more technical info into json obtained by "Export to json" action. This info could be then send in an issue report.

Example of the technical info:

1. browser version
2. current set of settings
3. how long metamath-lamp has been working since the page load
4. amount of currently used RAM (if it is possible to determine from JS)

There should be a clear explanation why users may want to include this info into the exported json. Also there should be possibility to specify what kind of info users don't want to include.

[david-a-wheeler]

This is a great idea! However, please make this off by default, to protect user privacy.