Make tests with vocabulary the user has not seen in a long time

[gabrielglbh]

We would need to add a new variable to Kanji: dateShown as an int in milliseconds. This variable for a certain word A should update everytime the user practices or makes tests with A in it. When validating A, the variable should update to DateTime.now().toMilliseconds().

This will be later used in a simple query to gather all words in the vocabulary ordered ASC and taking the first 30. In this way, the test will contain the 30 least seen words of the user's vocabulary, thus making the user remember those.

[gabrielglbh]

Basis done. Still needs to update tutorial texts and images, google play images.