Increase fidelity of conservation and delivery

Before, the thresholds for conservation and delivery mode was quite
coarse, immediately throwing someone into the most conservative bucket
as soon as their battery hit ≤20%. This change adds more granularity,
putting users into the ‘cautious’ bucket unless they have:

* critically low battery, or;
* Save-Data explicitly enabled, or;
* a poor connection.

Author: csswizardry