WE transition: I18n: convert *.properties to message.json files.

[myrdd]

Task:
convert all *.properties locale files (src/locale/*/requestpolicy.properties) to the WE message.json files.

Docs:

• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Internationalization
• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/i18n/Locale-Specific_Message_reference

Details:

• put the locale json files in src/conditional/webextension/_locales/*/message.json
• create symlink src/conditional/legacy/content/_locales with target src/conditional/webextension/_locales (relative target!)
• The .properties files contain many unlocalized strings. They are all duplicated in the non-en-US locale files. (For example, search for the string Learn more about rules. in all properties files.) All those duplicated messages can be removed, except in the en-US locale.
• keep the *.dtd files as they are! They are used only for the Request Log, which will not be ported for now, see WE transition: Request Log #872.

To be considered

• The .properties files contain many unlocalized strings, all being duplicated in each locale. I'm not sure if Mozilla implemented message "fallback" (see https://www.chromium.org/developers/design-documents/extensions/how-the-extension-system-works/i18n#TOC-Locale-fallback).

[myrdd]

I'm not sure if Mozilla implemented message "fallback"

Looks like they've implemented it. (See here, note the highlighted lines: https://dxr.mozilla.org/mozilla-central/rev/a461fe03fdb07218b7f50e92c59dde64b8f8a5b0/toolkit/components/extensions/test/xpcshell/test_ext_i18n.js#31,67,123,129-132)
So all those duplicated messages, that is, all english messages in non-english locale files can be left out in the corresponding message.json.

[myrdd]

I've created a branch with a first commit containing the symlink:
https://github.com/myrdd/requestpolicy/tree/feature/we-i18n

[myrdd]

Done. Thank you @beriain :) See myrdd@b97890e and #877