Standardize text colors across applications

We are currently using different text colors in:

1. The client (`text-color-text-light` / 0x20, aka. `text-grey-9`)
2. The pattern library (`text-slate-600` / 0x475569)
3. h's new UI (`text-grey-6` / 0x59)

Additionally we lack a clear set of conventions about the text colors to use for primary and secondary headings of various sizes. The current color used in h has a problem that it is difficult to differentiate placeholders from values in form fields. The pattern library color was an experiment at adding a slight blue tint, which is common.