Add reactions to slack connector

[jacobtomlinson]

Description

The slack connector doesn't support the react method yet. This PR adds that functionality.

Status

READY

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Manually tested with Slack and this skill gist.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #742 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #742   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     27           
  Lines        1793   1804   +11     
=====================================
+ Hits         1793   1804   +11

Impacted Files	Coverage Δ
opsdroid/connector/slack/__init__.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 61f52ef...87e67ca. Read the comment docs.

[FabioRosado]

Looks good to me 👍