Standardize language for braille codes and grades metadata

[wfree-aph]

There are many different ways that one could refer to braille codes and grades. Braille regions around the world don't even necessarily agree on the terminology or numbering system for different grades. An example of how the same braille code could be called different things is the following:

• Unified English Braille, Uncontracted
• Unified English Braille, Grade 1
• UEB, Grade 1
• UEB, No contractions
• UEB-1
• UEB-uncontracted

Some places we could use as a reference are World Braille Usage or LibLouis' braille tables. World Braille Usage seem to be out of date as it still lists the American code as English Braille American Edition, so it may not be a reliable source. LibLouis is not meant for that purpose, so it would be problematic to force it to be that resource.

Another solution is that we standardize it ourselves. This is not an easy feat and we are likely to miss some braille codes. We would need to make it clear that our list is not exhaustive. We could make a wiki and work on it together and then once we feel confident in our list, we could publish it to the specification, either as a part of the main spec or in a separate document that is referenced in the spec. The list will be long enough that I think it would need to be a separate document.

The other option is that we don't try to standardize braille code and grades and instead let this be decided at the publisher level.

[egli]

Related to #195