Improve or remove how "client" version of the application integrates with the extension.

[kaglowka]

The issue here is the "client" development mode of the application (whose product is a standalone JS script running on localhost).

On the one hand

• it is currently not used very often
• is more and more costly to maintain.

On the other hand

• in some circumstances it might still be very useful to develop parts of the application in a sandbox tuned to particular needs. To name a few:
  • tampering with window appearance without the windows disappearing on mouseleave
  • having full control over the document the script runs in
• the reloading of browser extension is and will probably always be more time-consuming and complex (and need manual reloading more often)
• React/Redux devtools cannot be used (at least out of the box) for, respectively, React component / Redux store in a browser extension content script.

I suggest that we soon either remove this development mode altogether or formulate specific needs we have to adapt it to these.

@glowka @KasiaSkowronska @lgodziejewski If you have any thoughts about our "sandbox needs" and how we can meet them in more details, please share! ;)

[kaglowka]

During a meeting we found the solution in #206 viable and the most flexible one, so we're currently opting for experimenting with it.