Improve protection against SSRF attacks
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.
- Tried to download macros from various URIs which issued redirects, etc.
- Downloaded an actual macro.
- Went through external account workflow.
Reviewed By: btrahan
Maniphest Tasks: T6755
Differential Revision: https://secure.phabricator.com/D12151