Adding logo assets (AcademySoftwareFoundation#235)

Adding here the finalized logo. This is almost identical to the one posted on Slack, except:

  - extremely small adjustment to the arrow head, to make it perfectly centered on and orthogonal to the stroke

  - SVG was optimized to remove redundant elements (e.g. gradients). Main logo is now only 10Kb. PNGs exported at 800dpi.

Author: portsmouth