Fix for #9925: Convert font data to BASE64, fixing 0xEFBFBD (Unicode Replacement Character) chars in customizer fonts.

First 16 bytes in original glyphicons-halflings-regular.ttf
    
    0000000: 0001 0000 000f 00[80] 0003 0070 4646 544d
    
First 16 bytes in customizer version:
    
    0000000: 0001 0000 000f 00[ef bfbd] 0003 0070 4646
    
This is caused by the 'utf8' encoding in <code>fs.readFileSync</code> function, which replaces any unknown unicode chars with the "EF BF BD" character (all throughout the font data).

The actual fix comes from this line:

          return type == 'fonts' ? files[path] = fs.readFileSync(type + '/' + path) :
        	  files[path] = fs.readFileSync(type + '/' + path, 'utf8')

But the <code>btoa</code> function decreases the raw-files.js by 200kB, so I let it stay.

[Fixes #9925]