* DEV: `Discourse.baseUri` does not exist
This never could have worked - should have been `Discourse.BaseUri` if
anything.
* DEV: Remove Discourse.Environment
* DEV: Remove `Discourse.disableMissingIconWarning`
* DEV: A bunch more missing environment checks
This reverts commit 20780a1eee.
* SECURITY: re-adds accidentally reverted commit:
03d26cd6: ensure embed_url contains valid http(s) uri
* when the merge commit e62a85cf was reverted, git chose the 2660c2e2 parent to land on
instead of the 03d26cd6 parent (which contains security fixes)
