Merge master to dev (#855)

* - Update tensorflow version to 1.15.0
- Add doc for Jetpack 4.4
- Remove opencv installation

* Add dataset and pre-trained model page

* fix issue #634 (#646)

- remove jquery dependency
- add clearTimeout

* Allow overriding WEB_CONTROL_PORT from env variable

* pin h5py version

* fix h5py version for mac

* * Update doc with donkey train command. (#709)

* Update doc with developer section for building own models in donkey 4.1
* Integrate changes from PR feedback

* Donkey 4.x is now official (#723)

* Update nano install docs - python3-opencv vs libopencv-python

* change slack to discord on readme

* Donkeycar 4.x release.  (#644)

## Major Improvements

- New Datastore.
- More ways to pre-process image data for training. 
- Use the 2.x version of Tensorflow. 
- Lots of other minor improvements.

* Improvements to the car app and handling of KerasPilot parts: (#648)

* Improvements to the car app and handling of KerasPilot parts:
*...