Add a difficulty level to capture the subjective feeling about the page

[siraj-samsudeen]

When entering the mistakes in the revision entry screen, add a set of choices to capture the difficulty level of the page - a qualitative feeling of the reciter/student or the listener/teacher about the page (irrespective of the quantitative data about the mistake). This field would be used to alter the next interval.

Values:

1. Easy
2. OK
3. Hard

[siraj-samsudeen]

Need to implement the calculations on the backend to use this new field