EQUATION EXPLORER: Allow searching/displaying any equation

[vlad902]

With Bruno's new fast algorithm for mapping equation to id and back, there's no reason the equation explorer can't handle arbitrary equation numbers or formulas, even if we don't have implications it could still be useful as a web-based find_equation_id.

[teorth]

This can be combined with #378.

[vlad902]

It could be, though they're really two separate tasks.

[blefloch]

Note that my python code is efficient in almost all cases, with the exception of equations with the same shape on the left-hand side and right-hand side: then it has to list all equations in order (basically). This could probably be solved if there is appetite for it, but the combinatorics of how to count equations up to isomorphism in that case were a bit involved.