Allow translators to define translator-specific hidden preferences.
Tweaks to getHiddenPref()

- Remove references to configOptions, displayOptions, and hiddenPrefs
  from connector/translator.js. We can't rely on them to be set in
  Zotero.Translator since https://repo.zotero.org/repo/metadata doesn't
  provide them, but we shouldn't need them until we get the translator
  code anyway.
- Don't serve configOptions, displayOptions, and hiddenPrefs from Zotero
  Standalone server. There's no point in serving anything that
  https://repo.zotero.org/repo/metadata doesn't.
- Alias the translator metadata loaded into the sandbox as
  Zotero.Translate._translatorInfo. Use this to get configOptions and
  hiddenPrefs.
- "v" browserSupport indicates that a translator can be run through
  translation-server, not that translation-server is required.

This obviates the need for zotero/zotero-connectors#4.