Reduce job status page refresh rate

[abought]

Purpose

The "job status" page provides feedback by refreshing every 5 sec.

When the queue is long, the server can experience loss of service quality just from status page checks alone. (see related connection pool issue)

The new proposed value of 20 sec will reduce server load by ~75% for running job status checks, while still giving users frequent visible feedback during "fast" parts of the process like QC.

Combined with other recent changes (DB indices, increased connection pool etc), this should be sufficient to prevent DoS when the site is used under normal conditions.

This will be included in our next fork release, and we're contributing it back for others.

Related:

• Hard-coded database connection pool size too small, requests failing #146
• Reduce polling times to prevent accidental DoS lukfor/imputationbot#33