Synapse uploads thumbnail data to S3

[rkfg]

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

[squahtx]

Splitting the url_cache aspect of this issue out into #10862