[Web] Remove dashboard daemon, deprecate flags

[jamesdphillips]

NOTICE: Upon release, consumers of the community edition of Sensu will need to migrate to the standalone dashboard. Instructions can found at https://github.com/sensu/web#getting-started.

What is this change?

• Removes the dashboard daemon from the backend
• Marks all dashboard related CLI flags as deprecated
• Removes the embedded dashboard assets. This saves around ~2MB from the sensu-backend binary.

Why is this change necessary?

In short the complexity of building and maintaining multiple distributions of the Sensu web interface has lead to slower development and a less than stellar community edition. By simplifying our build targets we hope to be able to serve both the community and commercial distributions better.

@calebhailey does the topic justice in Building A Better UI for Sensu; an excerpt:

Although it has taken various different forms over the past few years, the Sensu project has been committed to open source from day one. At some point along the way we adopted an open core business model 1, which has helped us continue to invest in the OSS project even as we have built a commercial product offering on top of that core engine. Most recently we made the decision to modify our distribution model so that 100% of Sensu Go’s commercial features are now free up to your first 100 nodes. This has had a pretty huge impact on Sensu adoption, simplifying the process of choosing which version of Sensu you should use when you’re initially evaluating the platform – we’re now seeing that almost 100% of Sensu installations with 100 nodes or fewer under management are running the commercial distribution.

While this has been great for users, it has complicated our OSS development process to a certain degree – primarily with respect to the Sensu Go web UI. Trying to maintain a common code base between OSS and the commercial version of the UI which leverages the commercial features of the Sensu backend has introduced some complexity that has slowed frontend product development.

This friction combined with the success of our commercial distribution strategy (i.e., FREE Sensu for everyone!) has prompted us to make the decision to split the OSS web UI off as a separate project. Moving forward, new web UI features will be implemented in the commercial distribution first and continue to be made available for free up to your first 100 nodes.

Does your change need a Changelog entry?

Yes.

Have you reviewed and updated the documentation for this change? Is new documentation required?

A good question.

How did you verify this change?

Ran test suite and ran manually.

[echlebek]