Add keyboard shortcut for displaying list of modernized scores

[craigsapp]

Add the letter m (or similar) on the frontpage to show only scores that have modernization filters. Pressing m again will show all scores again. Perhaps add a button to the navigation toolbar for this if it is of general use.

[craigsapp]

Note that this is similar to the i shortcut on the front page which lists all of the scores that have IIIF linkages. Also consider adding the IIIF logo or some scan-style logo to the navigation toolbar to make the list publicly accessible.

[craigsapp]

Implemented with commit https://github.com/craigsapp/website-polish-scores/commit/3815385f2086b836999bbedeca0b65641bc6f884

There is now an M and I button that show all "modern" or "iiif" scores on the browse page:

The buttons are mutually exclusive, so only one will highlight at a time.