First it was the Irish, then Portuguese, then Brazilians – for more than 150 years, Somerville, Massachusetts, absorbed wave after wave of immigrants. Today, hipster professionals are pouring into this city next door to Boston. Somerville rents have shot up as much as 50 percent in 15 years, and a two-bedroom apartment for under $2,000 in a shabby chic neighborhood is a rare find. A similar trend is playing out all over the country – from Boston and Miami to Los Angeles and Seattle – and it’s squeezing working- and middle-class families the most, according to the Joint Center for Housing Studies at Harvard University. ……