Restore prepopulated charset table, but this time with just the
encodings from the WHATWG Encoding Standard. Assigning a charset to
Zotero.Item::attachmentCharset runs the value through
Zotero.CharacterSets.toCanonical() automatically.
This migrates attachment charsets to the new canonical values, clearing any
that are unsupported.
Other legacy mappings could still be added back, as disussed in #760.
