Add custom theme for Keycloak login

[Carifio24]

This PR is a small update to add a custom theme for the Constellations login. This isn't a terribly key update but I think will be a nice improvement to make it less obvious that the login is being handled via a different service.

A few notes:

• We override the default Keycloak theme rather than creating one from scratch. This allows us to just update the styling of what already exists. What I've added here is quite simple - I've grabbed the green color and the Roboto Condensed that we use throughout the app as the primary text/highlight color, as well as our almost-black rgb(16, 16, 20) background, to try to keep some stylistic consistency. Ultimately this is more of a "get it set up" type of thing that should make it easier to update the styling later if we want to.
  • One idea I was thinking of for a somewhat more exciting design would be some astronomical image with the login card in front of it (maybe with a blurred background so that the image is partially visible through it?) Any sorts of styling thoughts are welcome
• I copied the template.ftl file from Keycloak's theme base so that we could change the favicon since this can't be done from theme.properties for a login account. The linked discussion mentions overriding the existing favicon, but in the Docker container the built-in themes are inside of a JAR file. Editing a JAR was obviously not a path that I wanted to go down, so I've just set the favicon to use a WWT assets URL. I also updated the title while I was at it.

@pkgw I'm not sure exactly how things are set up in the deployment, but we can insert this into the Keycloak Docker container in the right place via

# Run from the repository root
# The container needs to be restarted - I assume the set of themes is populated during setup

docker cp keycloak-themes/constellations <container-id>:/opt/keycloak/themes/
docker restart <container-id>

[pkgw]

Thanks for putting this together! The infra that sets up the Keycloak server lives here:

https://github.com/WorldWideTelescope/wwt-terraform-infra/blob/master/prod/constellations-keycloak-app.tf#L34

We could change this to use a custom-built image, but it would be kind of nice to be able to keep on using a stock image. I'd like to look into whether there's a plausible way to get the custom theme while still doing that.

[pkgw]

Here's the main info on starting up the stock image: https://www.keycloak.org/server/containers

We can set environment variables (the app_settings in the Terraform config) and the app_command_line. It's not looking like those knobs give us a way to add and use a custom theme, from what I've seen so far.