Add statuses to Submission elements to indicate whether it is spam

[nstCactus]

This PR fixes #58.

It:

• adds an isSpam column in the contactform_submissions table
• adds statuses to the Submission element type
• implements the unified element editor

This gives the ability to use the status dropdown in the element index to show only Not spam / Spam submissions or all of them.

The unified element editor :

• adds a metadata sidebar with the element ID, status & creation/update dates
• adds the ability to open the submission in the editor slideout from the element index
• voids the need for a controller

This PR needs #186, otherwise submissions marked as spam by the contact-form-honeypot plugin will be saved as Not spam.

[nstCactus]

Hi @rosskenney,

Did you get a chance to review this PR? I'm available to make adjustments if needed!

[DynamiteGoesBoom]

@nstCactus thanks for this and other PRs 🎉 We'll try and review these all soon and appreciate the help with these!