Adds a config flag, `MinervaNightMode`, and wires it up as a skin option
for enabling/disabling the night mode toggle (sold separately)
Additionally, allow this feature to be enabled via the
`minervanightmode` query param
For now, turning the flag on will result in the class
`skin-nightmode-clientpref-1` being added to the HTML element, but in a
later change this will be updated to switch values based on the
forthcoming client pref
Bug: T355118
Change-Id: I35848ab80ee75d324ceb35d17794e2d3e620cc19
The use of "HookHandlers" attribute in extension.json makes it possible
to inject services into hook handler classes in a future patch.
Bug: T315823
Change-Id: I9422f0cf7d5c14db544ea67c564d34e9eca3813b
