Make various sensor related updates

[caseycesari]

Overview

Updates the sensor healthy ranges based on feedback from the client, add fallback to sensor defaults in case of polling error, and add the "fair" calculation to the sensor overall rating logic.

See commit messages for details.

Connects #79 Connects #69

Demo

Testing Instructions

• Load the app, visit a sensor, and open the sensor detail modal.
• Verify the fish symbol for each variable is correct given the variable value and healthy range (outside of range = red, within 10% of edges = yellow, everything else = blue).
• Also verify that for the two failing sensors (Tinicum and Wissahickon), the default data is used.

[caseycesari]

Thanks. Pushed up a couple fixup commits to address your comments.

[caseycesari]

Thanks!