As a follow-up to the previous commit which replaces the spectrum color
picker instances with the custom color picker popover, this commit
removes the spectrum-colorpicker library and its related code from the
codebase.
This commit replaces the spectrum color picker used in the stream
popover and stream settings with a custom color picker popover, which
contains a grid of predefined color swatches and a custom color option.
The custom color option uses the HTML5 <input type="color"> which shows
the native browser color picker UI.
Fixes#14961.
