message: Add caching to `repo-age` feature
diff: import React from 'dom-chef';
  import select from 'select-dom';
  +import cache from 'webext-storage-cache';
  import timeAgo from '../libs/time-ago';
  import features from '../libs/features';
  import * as api from '../libs/api';
  import * as icons from '../libs/icons';
  -import {getRepoGQL} from '../libs/utils';
  +import {getRepoGQL, getRepoURL} from '../libs/utils';
  -async function getRepoCreationDate(): Promise<Date> {
  +const getRepoCreationDate = cache.function(async (): Promise<string> => {
  const {repository} = await api.v4(`
  repository(${getRepoGQL()}) {
  createdAt
  }
  `);
  - return new Date(repository.createdAt);
  -}
  + return repository.createdAt;
  +}, {
  + expiration: 3,
  + cacheKey: () => __featureName__ + ':' + getRepoURL()
  +});
  async function init(): Promise<void> {
  - const date = await getRepoCreationDate();
  + const date = new Date(await getRepoCreationDate());
  const {value, unit} = timeAgo(date);
  const element = (
  