Github SignUp/SignIn

# Problem

We want to add github signup/sign in in addition to the github connect we currently have.

This can be done followingly

## First, we add buttons to the UI

- Add a signup with github button to the signup page. This is basically the connect with github button on the user integration page, just with different test
- Add a sign in with github button to the login page, same as above

__Considering the behavior of the button is about the same, we should rename the existing component and then rewrite it so it yields to content (so we can specify text/content ourselves).__

## Next, we modify our github route

Right now, it sends the code and the state to our API and pushes the returned user into the store.

We should modify this so it also signs the returned user into the session and, if that happens, does not redirect to the `settings.integrations` route and instead redirects to the redirected from route (should be persisted in the session).

## That's it on ember

That should be all there is to it on the side of ember. We can expand later, by, for example, allowing users who signed up through github, to set their password, etc.

The rest is done on the API - https://github.com/code-corps/code-corps-api/issues/9#issuecomment-329185148

@JoshSmith Does this sound like it should be it, or am I missing something?

從使用者整合頁面上現有的 GitHub 連線按鈕開始，接著追蹤註冊頁面、登入頁面和 GitHub 路由。檢查該路由如何更新 store 和 session，包括持久化的重新導向來源路由，並查看關聯的 API issue 以了解伺服器端行為。當 GitHub 註冊和登入按鈕可以正常運作，且驗證成功後會重新導向至原始目的地時，即表示完成。

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