Release 2.21.0

18 new icons:

• backslash
• rewind-backward-10
• rewind-backward-15
• rewind-backward-20
• rewind-backward-30
• rewind-backward-40
• rewind-backward-5
• rewind-backward-50
• rewind-backward-60
• rewind-forward-10
• rewind-forward-15
• rewind-forward-20
• rewind-forward-30
• rewind-forward-40
• rewind-forward-5
• rewind-forward-50
• rewind-forward-60
• skateboarding

Fixed icons: cylinder-off