Allow to preview markdown content before sending comments and messages

[willemarcel]

We have some comment and message forms that supports markdown content. It would be good to allow the users to preview the content before posting, in order to avoid markdown formatting errors.

[wireguy]

associated markdown enhancements:

• have some helper text to show what markdown is available to use
  osm shows this, but you may not want to use the space (maybe a popup with text?)
  

• have a toolbar, like github or slack that allows formatting that gets converted to the markdown. This is great for users that don't know what markdown is or how to use it

[Aadesh-Baral]

@HelNershingThapa is this issue resolved by #5094?

[HelNershingThapa]

@HelNershingThapa is this issue resolved by #5094?

Nope, only the project detail comment section has the preview thing as of now.