Allow unicode accented characters in page names

[sbrl]

We really should allow page names to contain unicode characters. Currently, we try to avoid them in order to make sure that the filenames we use to save to disk are safe. There are 2 approaches here:

1. Find some method of escaping special characters in filenames (lots of work - we'd have to go over practically the whole codebase and change it all 😐
2. Draw up a list of unsafe characters that aren't allowed in filenames (not so much work).

[sbrl]

This shouldn't be too difficult actually. The tricky part is making sure that we don't miss any characters that shouldn't be allowed. The Wikipedia Page on filenames is a good place to start.