Modernisation - latest dependencies, re-indent JS, replace mobx decorators with non-decorator equivalents

[pdorrell]

Hi, I'm new to "modern" Javascript development & react & mobx, so I picked this project as an example to figure out how to set up my IDE (emacs).

It turns out the project is a bit out-of-date (I guess 3 years is a long time in Javascript land).

My pull request contains the following changes:

• Update to latest dependencies, replacing or removing deprecated dependencies
• Turn on source maps for development
• Re-indent Javascript code to 2 spaces per indent level (that's what I normally use, and that's what it says at https://standardjs.com/rules.html - is that OK?)
• Replace mobx decorators with non-decorator equivalents
• Fix warnings about filter values being updated outside of an action

There aren't any automated tests, but I have tested it manually: adding, completing, deleting & filtering todos.

Yours, Philip.