cmd/repo-updater/repos/conf.GetUpdateInterval() can return negative durations (panic: non-positive interval for NewTicker)

[sfllaw]

• Sourcegraph version: 3.3.0
• Platform information: N/A

Steps to reproduce:

1. In the site config, set

   "repoListUpdateInterval": 9999999999

2. Restart Sourcegraph

Expected behavior:

The repositories are updated every 9,999,999,999 minutes.

Actual behavior:

repo-updater panics because GetUpdateInterval returns a non-positive duration:
https://github.com/sourcegraph/sourcegraph/blob/89ac13c006da9145efb6d674157865d9643cef71/cmd/repo-updater/repos/conf.go#L9-L15

1. We should handle the case where v < 0. Conventionally, negative durations for intervals means to wait forever. This means that references to GetUpdateInterval should check this and not actually run their tasks.
2. We should handle the case where time.Duration(v) * time.Minute exceeds the maximum Duration. See proposal: time/v2: make Duration safer to use golang/go#20757