Add additional path validations to UploadsPipeline

What does this MR do and why?

This refactor adds some defense-in-depth. There should be no difference to end users.

I was looking at this code earlier in the week, and noticed that it called save_avatar before performing a check to validate the file wasn't a symlink. I couldn't exploit it though - BulkImports::FileDownloadService makes sure whatever we download is safe before UploadsPipeline gets around to using it. The AVATAR_PATTERN regex was pretty loose, and the checks for not-avatars also didn't seem to validate that file_path was in a tmpdir (just that the path matches DYNAMIC_PATH_PATTERN regex).

It seemed like an area where things might potentially go wrong in the future, so I reshuffled the existing checks and added one more (ensure the path is in a tmpdir and not attempting to traverse out). Now even a sneaky avatar or other file shouldn't be able to get past the checks.

FWIW we addressed a severity1 in this area of code at the start of 2022: Arbitrary file read via the bulk imports Uploa... (#349524 - closed)

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