[i18n] Add Ukrainian translation / Update Russian translation

[Teraskull]

Description

• Updated https://github.com/filebrowser/filebrowser/blob/master/frontend/src/i18n/ru.json to include new strings.
• Added ua.json with complete Ukrainian translation.
• Updated all languages to include the language selection dropdown.

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• DO make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
• DO make sure you are making a pull request against the master branch (left side). Also you should start your branch off our master.
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• DO make sure that related issues are opened in other repositories. I.e., the frontend, caddy plugins or the web page need to be updated accordingly.
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