Add export duration time

[Ed McDonagh]

As per imports

[Ed McDonagh]

Added new template tag to convert seconds into a natural time duration. Applied it to both the exports table and the patient size imports table. Refs #106, #21.

→ <<cset 90005314d6fa>>

[Ed McDonagh]

Export duration added to the family of csv exports and the NHSBSP version. Again, reports a shorter time than Celery does, but within a couple of seconds (fewer for smaller exports). Refs #106

→ <<cset c4bd2bc45b71>>

[Ed McDonagh]

Export duration populated in db field and displayed on exports table. Also new template formatting tag created to display the seconds in natural language. Fixes #106.

→ <<cset 6619a3a48a92>>