Add missing rejection handling for Promises (#7008)

* Add eslint-plugin-promise to detect uncaught rejections

* Move alert generation for errors to actions/alert

* Add missing rejection handling for Promises

* Use catch() instead of onReject on then()

Then it will catches rejection from onFulfilled. This detection can be
disabled by `allowThen` option, though.

app/javascript/mastodon/actions/accounts.js

@@ -103,7 +103,7 @@ export function fetchAccount(id) {
       dispatch(importFetchedAccount(response.data));
     })).then(() => {
       dispatch(fetchAccountSuccess());
-    }, error => {
+    }).catch(error => {
       dispatch(fetchAccountFail(id, error));
     });
   };