TIR band: custom range bounds, threshold refresh, shared title

[bjorkert]

Follow-ups for loopandlearn#774.

• Custom range bounds: very low and very high are clamped so they never overlap a custom range (very low below min(54, low), very high above max(250, high)). With a custom range such as 45–300 mg/dL, in range in the TIR band and the Stats screen now matches the statistics box.
• Threshold refresh: the home screen stats and TIR band refresh when the custom low or high threshold is edited.
• Shared title: UnitSettingsStore.timeInRangeTitle provides the Range Mode title for both the TIR band and the Stats screen, so Custom mode reads "Custom Range" in both places.