Load strings from external file instead of using hard coded values

[NurMarvin]

I've extracted the most important strings from all files and put them into the lang/en_US.json file.
There are two strings that are important and weren't extracted because I was unable to extract without breaking the sites functionality.
This is due to them having links in them that I would have had to either include in the translation file, which could cause breaks over time through e.g. method or URL changes or alternatively make multiple strings that only include the text parts, leaving the links out.
There probably also is a third way more elegant way of doing this, but I was unable to get something like that to work with my limited Vue.js knowledge.
There are also some other "hacky" ways I used for other translations that can probably be done more elegant.
This is only the first step to localization, I might implement the language switching mechanism too at some point, but I wouldn't mind if someone else did.

Closes #45

[NurMarvin]

Oh and I also separated some previously united things, e.g. the creating and starting messages in the video player, to make it easier for translators to make individual translations where necessary.

[NurMarvin]

Another thing, I missed some messages because I got quite sleepy over the course of making this and now I'm too sleepy to finish it, so would appreciate if someone else could do it once this is merged.

[darnfish]

This looks really good! I'll run a couple tests locally and then I'll merge this in if you're ready.

[jeda]

I will do some changes before merging it in.

[jeda]

#77 needs to be done before giving attention to this, to ensure it can be correctly translated.

Also that due to linting changes, conflicts should be resolved now.