(feat) Add basic support for a multi-column layout for custom dashboards

[brandones]

Requirements

• This PR has a title that briefly describes the work done, including the ticket number if there is a ticket.
• My work conforms to the OpenMRS 3.0 Styleguide.
• My work includes tests or is validated by existing tests.

Summary

This adds basic support for multi-column layouts in custom dashboards. Things this does not do:

• make it possible to configure widgets to take up different amounts of horizontal space
• allow changing dashboard layout depending on whether or not the workspace is open

Screenshots

[brandones]

Ping @ibacher @ZacButko @denniskigen @donaldkibet

[ibacher]

@brandones Nice! One small thing: how does this look with the workspace open?

[brandones]

@ibacher Bad, it looks bad. This feature is definitely "use at implementer's own risk." The site that is requesting it is not using workspaces yet.