Implementation of isPageAccessible getter for refactor

[jarmoza]

This PR addresses #261.

The implementation of this boolean getter is a bit more complex than most in our store. isPageAccessible is a tweaked version of pageAccessible from the previous store. It checks the conditions necessary for access to the given page passed to it.

The tweaks from pageAccessible reflect the current state of the implementation of the categorization page (where assessment tools and tool groups are to be reimplemented soon). Currently one should only need at least one category-linked column and for the 'Subject ID' category to be used at least once to progress to the annotation page. Secondly, since we do not yet have a good conditional metric for moving to the download page, a value annotationCount is used. (To move to the categorization page, a non-empty dataTable must be present.)

The unit tests implement check for moving to the categorization page, annotation page, and download page in two ways. First, with the sufficient input and then secondly without the sufficient input.

[surchs]

Looks great @jarmoza, thanks! I left two suggestions in the actual getter that I think might be more readable, take a look and see if you agree. But everything works well, so good to go.