Anet A8 Rear Frame Part (printable on 220x220 mm bed)

### Why?
When I was assembling my Anet A8 I used threadlocker on some of the bolts, but fumes from the glue caused the acrylic frame to crack over the span of the next few days. (It's known issue but I didn't knew about it at the time.)

So I found the broken part here on Thingiverse (it's file `A8-0009.stl` in **[Anet A8 acryl frame](https://www.thingiverse.com/thing:2288314)**), but it had different layout of some of the mounting holes, so I created another one from scratch (I actually boug