Display ISNI according to ISNI standard

[bencomp]

MOD 11-2 is defined in ISO/IEC 7064 and there's code available in a PyPI package https://pypi.org/project/python-stdnum/

As an aside, the draft of the ISNI standard, ISO/IEC 27729, that I found has the following about rendering:

When an ISNI is written, printed or otherwise presented in a human-readable format: a) it shall be preceded by the letters ISNI, separated from the identifier by a space, and b) the 16 digits shall be displayed as four blocks of four digits, with each block separated from the next by a space. EXAMPLE ISNI 1422 4586 3573 0476

OpenLibrary isn't currently following these rendering rules.

Originally posted by @tfmorris in https://github.com/internetarchive/openlibrary/issues/6841#issuecomment-1208348204

[agmckee]

This used to be formatted but was changed in #5364, specifically in the second commit in that pull request bf3ff608ba94d1db2d587af1f7317823e22ecab1.

[tfmorris]

Good spelunking! It looks like the regression wasn't caught during the code review, but seems easy to fix.