*: add theme based frontend configuration

This PR reworks the web layout so static files can be provided and
a "themes" directory to allow a certain degree of control over logos,
styles, etc.

This PR does NOT add general support for frontend customization,
only enough to allow us to start exploring theming internally.
The dex binary also must now be run from the root directory since
templates are no longer "compiled into" the binary.

The docker image has been updated with frontend assets.