Additional OAuth2 providers (#1010)

* add google+

* sort signin oauth2 providers based on the name so order is always the same

* update auth tip for google+

* add gitlab provider

* add bitbucket provider (and some go fmt)

* add twitter provider

* add facebook provider

* add dropbox provider

* add openid connect provider incl. new format of tips section in "Add New Source"

* lower the amount of disk storage for each session to prevent issues while building cross platform (and disk overflow)

* imports according to goimport and code style

* make it possible to set custom urls to gitlab and github provider (only these could have a different host)

* split up oauth2 into multiple files

* small typo in comment

* fix indention

* fix indentation

* fix new line before external import

* fix layout of signin part

* update "broken" dependency

vendor/github.com/mrjones/oauth/pre-commit.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# ln -s $PWD/pre-commit.sh .git/hooks/pre-commit
+go test *.go
+RESULT=$?
+if [[ $RESULT != 0 ]]; then
+    echo "REJECTING COMMIT (test failed with status: $RESULT)"
+    exit 1;
+fi
+
+go fmt *.go
+for e in $(ls examples); do 
+    go build examples/$e/*.go
+    RESULT=$?
+    if [[ $RESULT != 0 ]]; then
+        echo "REJECTING COMMIT (Examples failed to compile)"
+        exit $RESULT;
+    fi
+    go fmt examples/$e/*.go
+done
+
+exit 0