Fix bug: plugin scripts being added every page load

[garrettluu]

Closes #81.

Plugins are still registered with every rerender of App.js. Currently, the function call looks like this:

useEffect(() => {
    registerDefaultPlugins();
});

Changing it to this would theoretically fix the problem, since it would only be called on the first render:

useEffect(() => {
    registerDefaultPlugins();
}, []);

but for some reason the plugins all become undefined and everything breaks. (Is this bug even worth fixing?)

[barronwei]

What do you mean by the plugins become undefined?

[barronwei]

Can you try doing

  useEffect(() => {
    importDefaultPlugins();
    registerDefaultPlugins();
  }, []);

[garrettluu]

Probably gonna close this once #113 gets done