Change hosting from AWS S3 to GH-Pages

[thomaseizinger]

With this PR, we are changing the hosting of the frontend from AWS S3 to GH-Pages.

Motivation:

1. GH-Pages now offers free SSL-certs, also for custom domains.
2. The deployment is actually easier than I remembered, as Travis has now a pages deployment script.
3. Caching on GH-pages is better.
4. GH-pages serves files with Content-Encoding: GZIP out-of-the-box. Means we no longer have to pre-compress our files.

Things to do after this PR is merged and the website got successfully deployed:

• [x] Delete the AWS access keys from the travis build configuration
• [x] Delete the S3 buckets
• [x] Force redirect HTTP to HTTPS in settings.

Preconditions:

• [x] #118 is merged.
• [x] The cert is available.