Discord Oauth2 support

[techknowlogick]

Fix #4125

[codecov-io]

Codecov Report

Merging #4476 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4476      +/-   ##
==========================================
- Coverage   37.75%   37.75%   -0.01%     
==========================================
  Files         325      325              
  Lines       47638    47640       +2     
==========================================
- Hits        17988    17986       -2     
- Misses      27056    27061       +5     
+ Partials     2594     2593       -1

Impacted Files	Coverage Δ
models/oauth2.go	35.55% <ø> (ø)	⬆️
modules/auth/oauth2/oauth2.go	22.79% <0%> (-0.35%)	⬇️
models/unit.go	0% <0%> (-14.29%)	⬇️

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[lafriks]

Conflict

[jonasfranz]

If I try to authenticate via Discord, I'm getting this error:

2018/07/20 10:44:54 [...routers/user/auth.go:542 handleOAuth2SignIn()] [E] UserSignIn: oauth2: cannot fetch token: 401 Unauthorized
Response: {"error": "invalid_client"}

I've created a discord application and added the secrets. Then I've added the redirect link to OAuth2 at Discord.

[techknowlogick]

@JonasFranzDEV thanks for testing. I just tested and can confirm same results. Using same keys with goth example I was able to login, so it is 100% something wrong with my implementation. I'll go through and see what I did wrong.

[l0go]

yay!

[techknowlogick]

@jonasfranz I've update vendored dependency, and I am successfully able to login now, please review.