Synapse uploads thumbnail data to S3

This issue has been migrated from [#5411](https://github.com/matrix-org/synapse/issues/5411).

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### Description

Synapse uploads not only local media but also URL cache (including thumbnails for both local media and URL previews). It shouldn't because this data is either temporary or can be generated locally without wasting money on S3 requests and storage.

### Steps to reproduce

- enable S3 storage provider
- open the S3 bucket in AWS console
- see redundant directories like `url_cache`, `url_cache_thumbnails`, `local_thumbnails`

I expect to only have `local_content` there (optionally `remote_content` if enabled in the config).

### Version information

- **Homeserver**:

Own server

- **Version**:

0.99.2

- **Install method**:

From apt repo

- **Platform**:

Debian 9.9 amd64