Previously we used to mark a key as unstranlated if its value was equal
to it in translations.json. This had an issue because it didn't allow
otherwise valid cases where key was equal to the value.
This commit solves the problem by disallowing an empty string as a valid
translation and then using the empty string as the value for all the
unstranslated keys.
Fixes#5261
This replaces the `startsWith` string prototype method with `indexOf`
because no versions of Internet Explorer support this feature, and
it really is not difficult to just use `indexOf` instead and check
whether the starting index of the full string is 0.
The old translation copies in localStorage were not being removed
when they were no longer needed, so we can free up the storage
by deleting them.
This was accidentally not merged months ago when originally
implemented, but it was written to fix#4443 and in fact does so.
