resolves #136 and resolves #140

This PR adds proper validation to the classes.attach route. It ensures that class attachments do not create cycles in the class tree.
Validation works by first checking that the new parent of the class is not one of its descendants and then checking that the new children of the class are not one of its (new) ancestors. When a request fails validation, a detailed error message will be returned in the response.

I also made several other changes to improve the code.

1. I created a new format for connections (which I refer to as the "separated connections format") for allowing isDescendant() and isAncestor() to perform simple intersection operations instead of using recursion to iterate through each connection.
2. I made isDescendant() and isAncestor() more efficient. I also added the ability for them to find multiple descendants/ancestors at once and return them.
3. I improved the ClassParentTableSeeder. It now creates nice looking trees instead of long sticks. I also added a parameter that provides more control over the number of classes that appear at the first level of the tree (under the root).
4. I wrote a test suite for checking that validation in classes.attach works correctly.
5. I reduced the number of queries performed by classes.attach.