Increase backoff attempts/delay before marking feed invalid

[humphd]

Fixes #2332.

This does two things to help avoid marking (probably) valid feeds as invalid when they fail for some reason (e.g., temporary network outage):

1. Increases the backoff delay to 10 minutes
2. Increases the number of retries to 10

What this means is that if a feed fails, we'll do the following:

1. Try again in 10 minutes
2. Try again in 30 minutes
3. Try again in ~1 hour
4. Try again in ~2.5 hours
5. Try again in ~5 hours
6. Try again in ~10 hours
7. Try again in ~21 hours
8. Try again in ~1.7 days
9. Try again in ~3.5 days
10. Try again in ~7 days
11. Try again in ~14 days
12. Try again in ~28 days

After a month, I think it's safe to assume this site is not coming back online, and we can mark it invalid.