Manual analysis mode

[kasnerz]

Add features which will facilitate manual error analysis:

• [ ] generate a random sample from the dataset
• [x] bookmark interesting examples
• [ ] write notes

There are two ways to do this:

• easier: implement it in JS without any saving and force the user to export the data (the data would be lost after reloading the page)
• harder: implement it using some sort of cookies / sessions / user accounts (to keep the data even after the app is reloaded or closed)

[kasnerz]

Initial work done in #27

[oplatek]

#69 persistent favourites.