WASHINGTON — Oath Keepers extremist group founder Stewart Rhodes was sentenced on Thursday to 18 years in prison for orchestrating a weekslong plot that culminated in his followers attacking the U.S. Capitol in a bid to keep President Joe Biden out of the White House after winning the 2020 election.Rhodes, 58, is the first person charged in the Jan. 6, 2021, attack to be sentenced for seditious conspiracy, and his sentence is the longest handed down so far in the hundreds of Capitol riot cases.It’s another …