Resolve "do not mutate vuex store state outside mutation handlers" warnings

After enabling Vuex's strict mode in !918 (merged), a number of "do not mutate vuex store state outside mutation handlers" warnings started showing up. The worst offenders appear to be actions defined in src/webapp/src/store/modules/repos.js and src/webapp/src/store/modules/designs.js that directly modify state instead of committing mutations, which the docs and the internet agree is the correct way of modifying state.