Deal with long-running processes more gracefully

[sminnee]

There are 2 long-running processes:

• Crawl
• Import

These should be dealt with more gracefully using either the messagequeue or the queuedjobs module (or gearman).

• Clicking the button should queue a job and return instantly.
• Ajax should be used to return the status of the job within the UI, uploading periodically (every 10s or so?)

Note that the external-content module already has some integration with the queuedjobs module.