By Isabelle Hallé / news@seacoastonline.com PORTSMOUTH — “This Fourth of July is yours, not mine. You may rejoice, I must mourn,” Kevin Wade Mitchell said, reading the words of abolitionist Frederick Douglass to a full house at Strawbery Banke Museum Monday. In an event sponsored by the Black Heritage Trail of New Hampshire and the museum, community members gathered to take part in reading Frederick Douglass’ famous speech: “What to the Slaves is the Fourth of July?” Douglass, an escaped…