Add figured bass search filter

[craigsapp]

On the browse (home) page, add an "F" button to search for/in scores that contain figured bass.

[craigsapp]

Implemented with commit https://github.com/craigsapp/website-polish-scores/commit/60553477e7dd14e115c608fdb718bdaff73f81c0 and https://github.com/craigsapp/nifc-digital-score-server/commit/3f8ef4fbda63cd6bfe7ec5b8eca665b31aac8b91

Example:

Click on "F" on top left to limit search to scores that have figured bass:

There are 1374 scores containing figured bass.

Example at random:

https://polishscores.org/?id=pl-sa--58-a-i-58--001-003-dublet