Warning: componentWillMount has been renamed

Heya,

When devcards is used with newer versions of React, like 16.11.0, the following warning is displayed on the browser console:

```
Warning: componentWillMount has been renamed, and is not recommended for use. See https://fb.me/react-unsafe-component-lifecycles for details.

* Move code with side effects to componentDidMount, and set initial state in the constructor.
* Rename componentWillMount to UNSAFE_componentWillMount to suppress this warning in non-strict mode. In React 17.x, only the UNSAFE_ name will work. To rename all deprecated lifecycles to their new names, you can run `npx react-codemod rename-unsafe-lifecycles` in your project source folder.

Please update the following components: DevcardBase, HistoryComponent, ReagentInput

printWarning @ react-dom.development.js:12339
lowPriorityWarningWithoutStack @ react-dom.development.js:12360
ReactStrictModeWarnings.flushPendingUnsafeLifecycleWarnings @ react-dom.development.js:12524
commitRootImpl @ react-dom.development.js:25648
exports.unstable_runWithPriority @ scheduler.development.js:819
runWithPriority$2 @ react-dom.development.js:12131
commitRoot @ react-dom.development.js:24890
performSyncWorkOnRoot @ react-dom.development.js:24297
eval @ react-dom.development.js:12181
exports.unstable_runWithPriority @ scheduler.development.js:819
runWithPriority$2 @ react-dom.development.js:12131
flushSyncCallbackQueueImpl @ react-dom.development.js:12176
flushSyncCallbackQueue @ react-dom.development.js:12164
warnedAboutHydrateAPI @ react-dom.development.js:24391
dispatchDiscreteEvent @ react-dom.development.js:1443
```

This is similar to https://github.com/reagent-project/reagent/issues/445, addressed by https://github.com/reagent-project/reagent/pull/443.