One common place that this happens (for us) is on a local
Dropbox .dev.corp.dropbox.com instance, which can't be reached
by the Zulip servers.
This commit also:
* Fixes the test suite
* Properly previews /photos/ links
(imported from commit b4788b6236e7a9d390e1efc4673be34d9ba5e091)
Apparently the "inline" treeprocessor is what runs the inline
patterns. Also re-enable the rewriting-to-https support.
(imported from commit 2fde2c1f15217a784f26b16db25ee745f424f2f0)
