Support comments on undecidable properties

[ScriptRaccoon]

This PR resolves #82. To mark a property of a category as undecidable, in the bottom of the respective file with property assignments, insert an entry into the new category_property_comments table. For example, to say that "FreeAb is accessible" is undecidable, add:

INSERT INTO category_property_comments (category_id, property_id, comment)
VALUES
(
    'FreeAb',
    'accessible',
    'The question if this category is accessible is undecidable in ZFC. See <a href="https://math.stackexchange.com/questions/720885" target="_blank">MSE/720885</a>.'
);

in the bottom of FreeAb.sql.

This is then displayed as follows on the category detail page (just like a reason for a decided property):

These comments can also be used for properties that are not yet decided, or need further specification of a parameter. For example, the comment on M-Set that we need to specify M to decide if it is strongly connected now is directly associated with this property.