Implement student specific customizations

[Felienne]

As originally posted and debated here: #1370

With the implementation of #1317 and the improvements of #1363 we are able to customize classes on adventures, example programs and/or hiding levels. A great start, but we would also like to allow the customization on a student level instead of a class level. For example, enabling some adventures for specific students who are more advanced and reduce the amount of adventures for other students. This can be combined with additional features such as maximum amount of faulty code runs, amount of programs required to unlock next level. This should be designed and implemented in collaboration with teachers using the Hedy teachers section. Overview of implementation:

We keep the original class-level customization
We can select a student and create a "unique" customization, being prioritized over de class-level one
New page is implemented customize-student to create UI to make changes
We assume that a student is only in one class, in any other case we simply select the first one
We should be able to get an overview of all student-level customizations to enable editing and/or removing
When removed, the class-level customization should be used again
Not really liking to make the assumption that students are only in one class. They might want to keep their progress or programs from "older" classes or follow two classes at the same time. Not sure how we can best approach this issue. Maybe students can select their current class from a dropdown menu? I also like the idea that students are able to "escape" their class and simply browse the Hedy website without having the need to logout.