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 6055347 and craigsapp/nifc-digital-score-server@3f8ef4f

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