i18n: Offer Slovenian (sl) in the UI!

From a quick look at static/translations/messages_sl.json, this
looks to be completely translated! So it's time to offer it as a
setting in the UI.

Thanks very much to our kind volunteer translators.

Author: chrisbobbe

src/settings/languages.js

@@ -64,6 +64,7 @@ const languages: $ReadOnlyArray<Language> = [
   { tag: 'pt', name: 'Portuguese', selfname: 'Português' },
   { tag: 'pt-PT', name: 'Portuguese (Portugal)', selfname: 'Português (Portugal)' },
   { tag: 'ro', name: 'Romanian', selfname: 'Română' },
+  { tag: 'sl', name: 'Slovenian', selfname: 'Slovenščina' },
   { tag: 'fi', name: 'Finnish', selfname: 'Suomi' },
   { tag: 'sv', name: 'Swedish', selfname: 'Svenska' },
   { tag: 'tl', name: 'Tagalog', selfname: 'Tagalog' },

static/translations/messages_en.json

@@ -255,6 +255,7 @@
   "Russian": "Russian",
   "Serbian": "Serbian",
   "Sinhala": "Sinhala",
+  "Slovenian": "Slovenian",
   "Spanish": "Spanish",
   "Swedish": "Swedish",
   "Tagalog": "Tagalog",