Attributes aren't translatable

Problem/Motivation

Attributes aren't translatable, translations are shared amongst all languages.
This causes problems for example with aria-label when you want to use a different translated string for each language.

Steps to reproduce

* On a site configured with more than one language, with 'Custom menu link content' also configured as translateable, add a new menu link item.
* Enter some values in the menu link attributes fields
* Translate the menu link item, entering different values in the link attribute fields.
* Observe it says '(All languages) on the link attributes field.
* When you save, the original values will be overwritten.

Proposed resolution

* Make the individual attribute fields translatable.

Remaining tasks

User interface changes

API changes

Data model changes