Add Polish subtitles and route changes.

[JakubKoralewski]

What an absolute pleasure. Welcome. (II Iron Man 44:32)

I've added the ability to change from English to Polish and vice versa.
The supported routes are:

• for English:
  • '/en'
  • '/'
• for Polish:
  • '/pl'

This is my first project involving React so I'd appreciate any feedback in this regard.

For getting the subtitles in the correct format I used this: marvelsrt2json to generate the right format. I'd hope more people who know these languages can contribute!

The search placeholder is updated with the correct language as well.

[JakubKoralewski]

I already forgot but I also on load and on locale change generate a lowercase version of each movie so you don't have to do that on each search. I realise this misses the overall theme of this PR so I can remove it. The speed benefits are imperceptible, unfortunately.

[JakubKoralewski]

@seanlennaerts if you could review this; I'd gladly add more features, but I want to work on this branch though

[seanlennaerts]

Hey, so I've made quite a few changes but it'll also make adding new languages much easier. I'll update the readme over the next week to include instructions on generating the new sub files with an updated srt2json that you found before! Excited to add polish soon :)