Lift and shift the discussion block's code from edx-platform to xblocks contrib repo

[salman2013]

Lift and shift the following required files to load discussion xblock.

• XBlock python code
• Django templates
• JS files
• Css files

Acceptance criteria:

• Discussion block should properly load on cms and lms.
• Check the UI on cms and lms, everything is loading properly.
• Test cases should be green

[farhan]

On the way we will decide either to adopt webpack or django pipeline