Support colour disambiguation of users

[KitsuneRal]

A good portion of https://github.com/QMatrixClient/Quaternion/issues/513 (or https://github.com/QMatrixClient/Quaternion/pull/528, for the actual code) can be put to the library, so that clients could make use of it throughout the ecosystem without hitting the same rakes lying around in the grass.

[KitsuneRal]

What should be done here is to move the colour calculation from https://github.com/quotient-im/Quaternion/pull/528 to the library. The client should provide the palette, and the library would calculate and return the colour based on it.