Hide Preview tab when the toolbar with the preview icon is available

Currently, when the SE toolbar is enabled for a user, they get both: the Live Preview button on the toolbar, and the standard "Preview" tab. Both work differently which makes it very confusing.

It is suggested to approach the issue in several steps:

1. Connect the existing "Preview" tab to the live-preview functionality. The live-preview functionality of the SE has been tested for more than a year, and it should be safe to use as the main "preview" functionality. This will bring much better visibility to the feature as a side effect.
2. Roll out the source_editor_toolbar feature flag and make the toolbar available for all users
3. Completely remove the "Preview" tab. Since "Write" is going to be the only tab left then, we can drop both tabs in one go then.