This change adds support for displaying inline open graph previews for
links posted into Zulip.
It is designed to interact correctly with message editing.
This adds the new settings.INLINE_URL_EMBED_PREVIEW setting to control
whether this feature is enabled.
By default, this setting is currently disabled, so that we can burn it
in for a bit before it impacts users more broadly.
Eventually, we may want to make this manageable via a (set of?)
per-realm settings. E.g. I can imagine a realm wanting to be able to
enable/disable it for certain URLs.
- Replace download-zxcvbn with downloading it from npm.
- Change zxcvbn.js path to node_modules (because npm put it to
`node_modules` directory.
- Bump `PROVISION_VERSION` in `version.py` to 2.4.
Fixes#2423.
- Use tornado as proxy server for development environment,
replacing twisted (this doesn't support websockets).
- Upgrade tornado version to 4.4.1 (needs to be coupled to the
above since neither change works without the other)