Custom, per-user, default workspace image

Is your feature request related to a problem? Please describe

Gitpod uses workspace-full image as the default workspace images for repos which do not have .gitpod.yml file. I want to configure the default workspace image for all such repos at a central place.

Describe the behaviour you'd like

Given the user wants to configure the default workspace images for all their workspace without a .gitpod.yml file.
When the user navigates to settings menu.
Then they should see an option to configure default workspace image for all their workspaces.

Describe alternatives you've considered

1. Provide an option to the user to input the default image when user tries to create a workspace. However, this does not really solve the problem as they would need to input it every time.
2. Use users' github/gitlab etc user settings to infer the default image - This will add complexity as the user will need to manage the settings outside of gitpod.

Additional context

Based on initial request raised here https://github.com/gitpod-io/workspace-images/issues/680

Start by reading the linked workspace-images issue and reviewing the settings menu and workspace creation flow. Define how a per-user default image is stored and applied when a repository has no .gitpod.yml; the work is done when the setting is available and those workspaces use the selected image.