Issue 6743 - Deterministically assign actions to links in translations

This MR contains the following changes:

• Added support for <a0> to <a9>.
• Added support for <strong0> to <strong9> because it was simpler to implement this way and may turn out to be useful at some point. Please let me know if you want me to change the implementation or if you can think of any potential issues with allowing these.
• Moved setLinks() from common.js to i18n.js for consistency.
• Replaced javascript:void(0); link fallback with # to avoid any inline script-related issues.

See https://gitlab.com/eyeo/adblockplus/adblockplusui/issues/86