Allow non-unique anchors between languages

[iMattPro]

Currently every anchor must be unique.

We should allow non-unique anchors between different languages. So, for example, the anchor rule-1 will link to "Rule 1" no matter which language the user is viewing the rules in.

An example scenario is where a user is using English, and he copies the link to a rule and gives the link to another user. But the other user is on another language, so the link currently won't work for them since in their language, the rule's anchor does not exist.

So we should only enforce unique anchors within the current language a rule is being created in.

Based on this report:
https://www.phpbb.com/customise/db/extension/boardrules/support/topic/153026