i18n strings audit (#1350)

* Tried to get accessibilityHints to conform to Apple guidelines
* Tried to make some more helpful `accessibilityLabel`s
* Used fork of fluent_conv to support function results as selectors in Fluent
* Custom VOWORCON function for Fluent

Note that there are still many accessibility problems for screen readers. This
just tries to make sure the strings up for translation are somewhat less
confusing in that context.

Part of #1223

e2e/signedIn.e2e.js

@@ -145,7 +145,7 @@ describe( "Signed in user", () => {
     await element( by.id( `ObsDetails.${uuid}` ) ).scrollTo( "bottom" );
     const comment = element( by.text( "This is a comment" ) );
     await waitFor( comment ).toBeVisible().withTimeout( 10000 );
-    await element( by.label( "Navigate back" ) ).tap( );
+    await element( by.label( "Go back" ) ).tap( );
     await waitFor( username ).toBeVisible( ).withTimeout( 10000 );
 
     /*