fixed background white flash flash + changed transaltions

[quentinlegot]

I did many change to the project:

• I fixed the background white flash when changing image by loading it in js before trying to display it
• I refactored the project to be more readable (sry if you don't like my organisation but I think maintaining the project will be easier now)
• I changed the way the translation of "by" work to avoid future issue like here: https://github.com/busybox11/NowPlaying-for-Spotify/pull/38/files#diff-bb0d84ea0ecaa031ff330d408ae7f53c96223bf64679c07562f3b2c8358f00a9R12

Finally, I don't mark the PR as ready cause I didn't test last commit changes dut to an issue I have with apache, I'll mark it as mergeable asap

[quentinlegot]

I fixed the issue by deleting php_flag and php_value to .htaccess (I don't know why that cause issue now and not when I started debugging but ok) and fixed 1 issue cause I forgot some ";" the PR is now ready to be merged

[busybox11]

You're a beast. Thank you a lot!