local_content and local_thumbnails require case-sensitive file system

This issue has been migrated from [#5819](https://github.com/matrix-org/synapse/issues/5819).

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These two paths contain folders with two-character names which collide on a case-insensitive file system. Were about to make a backup of the Synapse data directory, but couldn't easily for this reason.

- /var/lib/matrix-synapse/media/local_content/{AB,Ac,AD,**aE,Ae,AE**,Af,**aH,Ah,AH**,...}
- /var/lib/matrix-synapse/media/local_thumbnails

I don't have any insight into why or if it makes sense.

Start by tracing where the media/local_content and media/local_thumbnails directories are created and accessed; no source files or tests are identified in the issue. Reproduce the reported collision on a case-insensitive filesystem and establish the expected layout and migration behavior before making changes.

Written by the indexing model from the issue text.