improved oauth success user experience flow

[stevengill]

Summary

Fix for issue #1010.

It will now show a generic success page which will automatically launch the redirect to the slack app. It also includes a button to launch the slack app incase users have javascript turned off or this click no during the original popup window.

This is an improved flow vs the allow permissions tab staying open with a spinning button

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[stevengill]

Let me know if the redirect message should be tweaked.

[aoberoi]

This is a good improvement! I can imagine folks wanting to update the styles on the success page to look prettier. Maybe we can update or extend the sub-task in the example for customizing the success handler to show how someone could render their own HTML page instead of only showing res.send('success')? I also just noticed that res.send() assumes use of Express. Should it be res.end()?

[stevengill]

Nice catch! I'll update that example part of this PR.

[codecov]

Codecov Report

Merging #1011 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1011   +/-   ##
=======================================
  Coverage   94.40%   94.40%           
=======================================
  Files          12       12           
  Lines         768      768           
  Branches      173      173           
=======================================
  Hits          725      725           
  Misses         14       14           
  Partials       29       29           

Flag	Coverage Δ
#eventsapi	89.61% <ø> (ø)
#interactivemessages	95.04% <ø> (ø)
#webapi	96.36% <ø> (ø)
#webhook	87.50% <ø> (ø)

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