When users choose a default project icon, make a permanent file

Summary:
Fixes T10907. As written, this workflow will incorrectly reuse a temporary file if one exists.

Instead, make a new permanent file.

(Storage is still shared, so this usually will not actually create a copy of the file's data.)

Test Plan:

• Set a project's icon by clicking first button in "Use Picture" row.
• Before patch: temporary image was reused.
• After patch: new permanent file is generated.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10907

Differential Revision: https://secure.phabricator.com/D16270