Add configuration settings to set hydrate reminder chat icon

[jmakhack]

Note: This is dependent on #355 so work cannot start on this issue until #355 is first completed.

Feature Reasoning

Given that there exist new settings to set the overlay timer background icon, it would be nice to have the same settings for the hydrate reminder chat icon.

Feature Description

The same image category and image settings that exist for the overlay timer should be ported over for the notification settings to set the hydrate reminder chat icon. The default icon should be what it currently is set to which is the water rune icon.

Additional Context

Please refer to CONTRIBUTING.md for info on how to setup, build, run, and test this project.