luci-theme-bootstrap: add explicit dark/light mode selection

Register two further "virtual" themes called BootstrapDark and
BootstrapLight which force dark and light mode respectively.

The actual Bootstrap theme itself will continue to auto-select
dark mode preference based on OS/Browser preference settings.

Fixes: #5492
Signed-off-by: Jo-Philipp Wich <jo@mein.io>