Visit generator should ask for user confirmation before generating lots of fake data

[willemijns]

The visit generator UI currently requires Super User access, but this is not enough, as it is available through a simple link in the API page. Instead, the UI should warn the user and ask for confirmation when clicking on the visit generator link. Once data has been generated, it is not possible to remove this fake data, which might corrupt historical reports and cause data issues.

[vipsoft]

The data generator is intended for testing and evaluating Piwik. This is documented in ./README.

[matt]

willemijns, if you can provide a patch for generateVisits and we would commit it.