Take GitHub "reactions" into account for "thumbs up" in issues

Issues on Bountysource have a "thumbs-up count", which (as described by rappo) is a combination of "1) vote counts (if applicable, eg Jira or Launchpad) 2) thumbsup comments in the thread itself (eg `:+1:` in the github thread) 3) using our browser extension, or 4) clicking the thumbs up button on the front end".

GitHub now has a way to attach "reactions" (a fixed set of emojis) to issue comments, notably including the description of the issue itself. A :+1: reaction on the issue description could be considered a vote, and fall under 1) in the list above, but this is not currently taken into account by Bountysource and it should be.

Note that [retrieving reaction information from GitHub](https://developer.github.com/changes/2016-05-12-reactions-api-preview/) is still in developer preview, which means the API isn't yet stable and may break in the future.

Start by reviewing the GitHub reactions API preview linked in the issue and the existing GitHub thumbs-up or vote integration. Add support for treating a :+1: reaction on an issue description as part of the thumbs-up count, then verify that the resulting count matches the stated Bountysource rules.

Written by the indexing model from the issue text.