Add support for client basic auth for exchanging access tokens

[jonasfranz]

Adds support for basic auth for exchanging access tokens. This is important since golang's oauth2 library requires that.

https://tools.ietf.org/html/rfc6749#section-2.3.1

[lafriks]

Test seems to fail with empty client id

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@e0eb651). Click here to learn what that means.
The diff coverage is 39.13%.

@@            Coverage Diff            @@
##             master    #6293   +/-   ##
=========================================
  Coverage          ?   38.87%           
=========================================
  Files             ?      358           
  Lines             ?    51038           
  Branches          ?        0           
=========================================
  Hits              ?    19839           
  Misses            ?    28329           
  Partials          ?     2870

Impacted Files	Coverage Δ
routers/user/oauth.go	38.88% <39.13%> (ø)

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[gsantner]

@jonasfranz / @zeripath
Hey!
I'm running the daily version git-ca46385, tried to backup, but can't due missing tables.
Guess theres DB migration missing or like that?

gitea dump -c ~/gitea/custom/conf/app.ini 
2019/03/19 17:06:11 Creating tmp work dir: /tmp/gitea-dump-640591686
2019/03/19 17:06:11 Packing dump files...
2019/03/19 17:06:11 Dumping local repositories.../home/git/gitea/data/repositories
2019/03/19 17:07:39 Dumping database...
2019/03/19 17:07:40 Failed to dump database: Error 1146: Table 'gitea.o_auth2_application' doesn't exist

[jolheiser]

Just a note for whoever looks at this, since I don't know enough about Xorm's backend.
The OAuth2Application struct is stored as oauth2_application which is why it can't find o_auth2_application when dumping.

gitea/models/oauth2_application.go

Lines 43 to 46 in e777c6b

	// TableName sets the table name to `oauth2_application`
	func (app *OAuth2Application) TableName() string {
	return "oauth2_application"
	}

[jonasfranz]

That's not caused by this pull request. Please open up a new issue to better manage the bug.

[gsantner]

#6385 / #6310