Display comments using their markdown format

[Cleop]

Currently we are displaying comments as text only and not accounting for line breaks, bullets, titles etc. that might be added using markdown.

As a user I want to see my comments looking the same as they do on github So that I have a smooth user experience between the two and can read the comments easily.

• [x] Add earmark to help display markdown comments with their github formatting.

[Cleop]

Markdown that does work with Earmark:

I think this is good as a first pass, I will create a separate issue for dealing with checkboxes as I think this is a lower priority for the time being.

[Cleop]

I've created #73 as there is a small padding issue, however testing of this issue can be done to assert that markdown such as bold, titles and images are being loaded correctly.

69 has been created to deal with the rendering of checkboxes.

[nelsonic]

@Cleop please provide direct link to test this on Heroku as master is not working on my localhost.

p.s. we could consider using "review apps": https://github.com/dwyl/learn-heroku/issues/16

[Cleop]

@nelsonic you won't be able to test this on staging unfortunately until #63 is solved. At present staging is not receiving any webhook events as it's not being pointed at from our github app (our local ngrok link is).

Shall I put a blocker label on this one?