By Gabriella Borter (Reuters) – Mayors from across the United States gathered in Washington for their annual winter conference this week to tackle major issues facing their cities, with mental health, addiction and mass migration high on their lists. In the wake of the COVID-19 pandemic, mayors have been on the front lines as their cities struggle to revitalize business districts decimated by a shift to working from home and confront an explosion in mental illness and economic woes. Mayors from Tampa, Florida, to Tacoma, Washington, were meeting with federal officials including President Joe B…