Awesome job, Ruonan! Looking great! 

I'm also learning about OAuth right now, so I'll definitely check this out

locally at a later time.

Needs an empty line between line 7 and 8 to separate 3rd party modules and current project codebase

Delete 'webhook URL'

I could be totally wrong, but was it not already in utf-8?

Remove print statement

Update Trello to Slack

Maybe use from django.utils.six.moves.urllib.request

Awesome Work!

Unfortunately, I experienced 28 test failures, although it is possible that I may have done something wrong when applying the patch.

Due to these test failures, I refrained from doing any manual testing.

I imported all the files, ran ./setup develop then rbtext test -m rbintegrations.slack (For the record I am also using RB4).

Please let me know if I did something incorrectly on my end.

I believe that python files, do not follow the camelCase naming scheme.

Include a line between lines 2 and 3.

Lines 10 and 11 should be swapped to maintain alphabetical order.

Include another empty line below line 10.

I noticed that you are using var here, but are not redeclaring the variable, perhaps this should be const.

I noticed you are experiencing some extra whitespace errors. I find that these can be hard to find unless you enable whitespaces in your text editor. Then they are easier to find and remove.
Similar, it does not seem like we are redeclaring win here so perhaps we should use const.

I could be wrong, but this function does not seem to follow camelCase styling.

I think we should be using let here.

Instead of making another variable param, I think that we can just return urlObj.searchParams.get(name);

I could be wrong about this as well, but I think we could use let here instead of var

This variable does not seem to be redeclared, so therefore I think it should be const.

Perhaps it would be useful to console.log or log the error somehow to make the error easier to determine when debugging in the future?

Please add a module docstring at the top of this file.

Let's use django.util.html.format_html here:

return format_html('<button onclick="authorize(\'{0}\')">{1}</button>',
                   server,
                   _('Authorize'))

We should wrap "Authorize" in a gettext call and format it into the HTML.

Add a blank line between these two.

Add a blank line between these two.

Please use single quotes here.

Instead of +, can you use a format operation?

These two lines will need to be swapped.

Can we add some blank lines in here to space things out a bit?

Why are these commented out?

This doesn't appear to be used.

Add another blank line here.

Please add a period in this docstring.

This needs a better docstring.

There's an extra blank line here that can go away.

For this, please use (at the top), from django.utils.six.moves.urllib.parse import parse_qs and then just call parse_qs.

This file should be named .es6.js, since you're using ES6 features.

This can use an ES6 template string:

const redirect = `${redirectServer}rbintegrations/slack/oauth/`;

Please use single quotes throughout.

This can also be a template string.

You've got some trailing whitespace throughout this file.

This can be const.

Instead of the magic number 4, can we compare against XMLHttpRequest.DONE?

We can get rid of the warning here, and make things more correct and reliable wrt. babel-compilation by assigning this to the window object. Let's also make the name more specific:

window.authorizeSlack = function(redirectServer) {
  ...
}

There's still some trailing whitespace within this file. Please look at the diff here on Review board and get rid of them.

console.error is probably more approriate than console.log here.