Fixes translation failing with "Google Translate HTTP 429".

Google stopped serving the endpoint parameter this add-on relied on, which broke
translation for everyone. This release switches to a working endpoint parameter
and makes the add-on far more resilient:
- Failed requests are now retried automatically with a growing delay, instead of
giving up on the first error.
- Messages are sent in fewer, larger requests, so a long email is much less
likely to hit a rate limit. Non-Latin scripts (Chinese, Japanese, Cyrillic,
Arabic) are handled correctly when sizing those requests.
- If the service does rate-limit you, you now get a clear explanation in your own
language instead of a raw "HTTP 429" error.

No new permissions.

The add-on name is now the same in every language ("Web Translate Toggle for
Thunderbird") to fully comply with Mozilla's trademark naming guidelines; the
interface and description remain translated. No functional changes.