Ec refactor

I finished refactoring the code for elliptic curves over Q.  It is now a lot cleaner, uses classes for (web) curves and (web) isogeny classes.  Little change to the appearances of web pages:  I did change the isogeny graphs appearance a bit (plotting with the "spring" option has been removed so now the shape is much more restrained).  Added a Properties box for isogeny class pages (which features a mini-version of the isogeny graph).

One general utility was added (in utils.py) making it much easier to include plots on pages, storing only a base64-encoded version of the .png file, created with no use of temporary files.