Implement 'filesizeformat' filter

Implement 'filesizeformat' filter according to the [jinja2 specification](https://jinja.palletsprojects.com/en/2.10.x/templates/#filesizeformat):

> Format the value like a ‘human-readable’ file size (i.e. 13 kB, 4.1 MB, 102 Bytes, etc). Per default decimal prefixes are used (Mega, Giga, etc.), if the second parameter is set to True the binary prefixes are used (Mebi, Gibi).