rename document_version => viewer

app/components/viewer/base.hbs

app/components/viewer/base.js

app/components/viewer/index.hbs

app/components/viewer/left.js

app/components/viewer/page.hbs

app/components/viewer/right.js

app/components/viewer/thumbnail.hbs

app/components/viewer/thumbnails.hbs

app/routes/authenticated/document.js

app/templates/authenticated/document_version.hbs

app/templates/authenticated/viewer.hbs