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: T6755

Differential Revision: https://secure.phabricator.com/D12151