WASHINGTON — A unanimous Supreme Court ruled Monday that Boston violated the free speech rights of a conservative activist when it refused his request to fly a Christian flag on a flagpole outside City Hall.Justice Stephen Breyer wrote for the court that the city discriminated against the activist, Harold Shurtleff, because of his “religious viewpoint,” even though it had routinely approved applications for the use of one of the three flagpoles outside City Hall that fly the U.S., Massachusetts and Boston …