R1:f1f8ee8e6ad5

Improve subproject/milestone error handling for users who can't create projects

Summary:
Fixes T10357.

- Show a better (more descriptive) error message when a user who can't create projects tries to create a subproject or milestone.
- Disable the subproject actions if you don't have create permission.

All this stuff was already enforced properly: this diff doesn't make any actual policy changes, just improves the UI for users who lack permission.

Test Plan:
- As an unprivileged user (no "Can…