Add logout button.

src/modules/users.js

@@ -26,6 +26,9 @@ export const mutations = {
   setCurrentUser (state, user) {
     state.currentUser = merge(state.currentUser || {}, user)
   },
+  clearCurrentUser (state) {
+    state.currentUser = false
+  },
   beginLogin (state) {
     state.loggingIn = true
   },
@@ -66,6 +69,11 @@ const users = {
         store.commit('setUserForStatus', status)
       })
     },
+    logout (store) {
+      store.commit('clearCurrentUser')
+      store.dispatch('stopFetching', 'friends')
+      store.commit('setBackendInteractor', backendInteractorService())
+    },
     loginUser (store, userCredentials) {
       return new Promise((resolve, reject) => {
         const commit = store.commit