R1:4f8147dbb8c0

Improve protection against SSRF attacks

Summary:
Ref T6755. This improves our resistance to SSRF attacks:

- Follow redirects manually and verify each component of the redirect chain.
- Handle authentication provider profile picture fetches more strictly.

Test Plan:
- Tried to download macros from various URIs which issued redirects, etc.
- Downloaded an actual macro.
- Went through external account workflow.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks…