Add separate cache directory for non-local uploads

[Gargron]

The following resources will be split into a top-level cache directory:

• custom emojis
• avatars
• headers
• media attachments
• preview cards

...when they don't originate from local accounts. With the exception of preview cards, which are always counted as not local. The change comes into effect for anything uploaded or downloaded after the upgrade, old files remain in place, so as to not incur massive object storage costs moving potentially terabytes of data.

Adds tootctl upgrade storage-schema to upgrade all existing files at once.

[ClearlyClaire]

This is a very welcome change overall, although I have some minor concerns (see inline comments).

[zunda]

This seems to have broken path to existing icons and custom emojis. E.g., my avatar which has been t https://s3.amazonaws.com/zundan-mastodon/accounts/avatars/000/000/001/original/89de63b40ca41c7e.gif is now rendered as https://s3.amazonaws.com/zundan-mastodon/system/accounts/avatars/000/000/001/original/89de63b40ca41c7e.gif_prefixaccounts/avatars/000/000/001/original/89de63b40ca41c7e.gif

git revert c3ca3801f fixed the problem for now. Not sure if related or not, my server is on ruby-2.7.1, makes direct use of S3 in us-east-1.

[mayaeh]

This seems to have broken path to existing icons and custom emojis.

This issue also occurs in my environment using Ruby 2.6.6 with OpenStack Swift.
This issue doesn't seem to occur on instances that are not using object storage.

[mayaeh]

This seems to fix it, but I don't understand why this problem occurs...

[orlea]

Is it the same as the old issue below?
#6675
#12382