Support asynchronous queries

[alexanderkoller]

For stability, we use the Tornado server with Flask in production right now. Unfortunately, Tornado in this configuration can only handle one HTTP request at a time. This is really bad, now that bulk email sending with the Mail Tool and nontrivial delays between messages can take a long time. During this time, all other HTTP requests will be delayed until the Mail Tool is finished.

Figure out a way to make long-running requests, especially in the Mail Tool, asynchronous. This may involve replacing Tornado by another server. Some options are listed here.

When this is done, remove the warning message from the Mail Tool page.