OpenAstroTracker - Digital Level Holder

I wanted to add the Digital Level Addon to my OAT, but i did not like the position of the MPU6050 sensor in the [official project STL-file](https://github.com/OpenAstroTech/OpenAstroTracker-Addons/blob/master/DigitalLevel/STL/Digital%20Level%20Holder.stl) (too far at the back, so longer cable runs etc).

In this remix, I moved the MPU6050 sensor to the front and changed the cable duct to work nicely with the [MKS + LCD case](https://github.com/OpenAstroTech/OpenAstroTracker-Addons/tree/master/Enclosures/MKS%20%2B%20LCD%20case). Additionally, I removed some material at the back of the holder to save approx 20% of filament and added a lid.

In order for everything to fit nicely, make sure to:
- Glue the sensor in place with a bit of superglue. I also did this for the wires on the front-facing edge of the holder.
- Use lens head screws on the lid of the MKS case (M3x8 worked fine for me).
- Attach the MKS case below the OAT using the provided MKS-case-spacer.STL using M4x30 screws