Add "Deploy to Azure" button

[hoisjp]

I added "Deploy to Azure" button which deploys Azure App Service (free plan by default) with one click action.

If a base repo can accept this merge, please tell me. I will change the forked URL to the base URL before merging. e.g. https://github.com/hoisjp/line-liff-starter => https://github.com/line/line-liff-starter

[smwilk]

@hoisjp Thanks for your patience. I was able to see the "custom deployment" screen, but I kept getting a "Cannot find SourceControlToken" error. It seemed like one's Github account needed to be linked to Azure, and the linked repository's URL had to be specified in the "Custom Deployment" page. Following the below steps allowed me to successfully deploy the app:

1. (First time only) Register one's Github account under "Deployment Center" in Azure portal.
2. Fork the repository.
3. Click the "Deploy to Azure" button in the forked repository.
4. Set the forked repository URL in "Repo url" under SETTINGS (e.g. https://github.com/{your-github-username}/line-liff-starter ).
5. Fill out the rest of the blanks as outlined here, and click "purchase".

Do you think there could be a workaround to this?