Update themes.js normalizing, removing redundant data

[raszpl]

can remove

"default_theme": {
    "message": "default"
},

from all locales

[ImprovedTube]

can do that with our py/locale.py -remove default_theme and/or crowdin or @unnamed-orbert
after checking each translation. Possibly some might translated default_theme, but didn't translate default yet

[unnamed-orbert]

Update the locales/en to me, to sync all others files! @ImprovedTube

[ImprovedTube]

Possibly some might translated default_theme, but didn't translate default yet

..and we can first rename default_theme to default with py/locale.py -change-key, if we make that feature not overwrite anymore or ask

youtube/py/locale.py

Line 115 in 8e41763

if old_key in data: