Related issue: #188

This is a work in progress PR. Lot of things to implemented/handled.

Done:

1. Show/submit reactions for existing issue
2. Show/submit reaction on new issue created by first comment
3. Show total reaction count

Pending things on top of my head:

1. Fix error when reacting on new issue created through a reactions button
2. Fix issue with updating the reaction count
3. Disable when user is not logged in
4. UI for showing "signin to react"
5. Style for various themes

Currently it looks like this:

- bug exists in new issue created through reaction buttons

I have pushed changes for:

1. Fix error when reacting on new issue created through a reactions button
2. Fix issue with updating the reaction count
3. Disable when user is not logged in

For, 4. UI for showing "signin to react":
Just a thought - there is a "Sign in to comment" already, we could rename it to "Sign in" and avoid creating another Sign in button. If you think a separate Sign in button is required for reactions, I would need your input on where to place the the Sign in button.

For, 5. Style for various themes:
I tested this. It appears to be working fine.

New screenshot:

The reaction buttons are getting wrapped to a new line unevenly and there is some issue with border as well in small screens. Let me know you thoughts on handling this.

I think that the reactions button should be in a popup menu on the far right side of the line that says "X Comments - powered by utteranc.es".

@arunvelsriram Will you continue working on this topic? I'd really like to see this feature coming.

Functionality wise everything is covered as far as I remember. There are UI improvements and we need to decide about the Sign In button.

I don't have much experience in UI development. Please fee l free to add improvements on top of this.

@arunvelsriram, you did fantastic work with this feature! It works like DISQUS. Pretty cool!

Is the Sign In button issue a must-have requirement? Can this PR be merged without this, and then another PR be created to solve the button UX, if required? Emoji Reactions can be enabled as a "beta" feature, let's say through the JS.