Stop checking what characters are in passwords, refs #1371, fixes #1372

#285 changed password validation from `alpha_numeric` to `alpha_dash` to solve a problem with the validation message, but (functionally) all this did was add the characters `_-` to the allowed list, which still prevents special characters from being used.

As noted by @rjmackay on #285, the real goal is to allow **any** characters to be used for passwords. (This is shown by #1371.) Right now, only letters, numbers, dash, and underscore can be used, but the restriction is arbitrary, as the end result is that all passwords are hashed, making the character set irrelevant.