Replace previewState with `import VIEW_MODE_STATES`

[sabina-talipova]

Description

• Replace previewState with import { VIEW_MODE_STATES } from 'state/viewMode/ViewModeStates' in HistoryViewerVersionDetail and HistoryViewer Components
• If that's not already being added via webpack externals, add it.

See:

• silverstripe-versioned-admin/client/src/components/HistoryViewer/HistoryViewer.js

  Lines 412 to 413 in f1492aa

  	// @todo replace this with import { VIEW_MODE_STATES } from 'state/viewMode/ViewModeStates'
  	// when webpack-config has this export available via silverstripe/admin

• silverstripe-versioned-admin/client/src/components/HistoryViewer/HistoryViewerVersionDetail.js

  Lines 229 to 230 in f1492aa

  	// @todo replace this with import { VIEW_MODE_STATES } from 'state/viewMode/ViewModeStates'
  	// when webpack-config has this export available via silverstripe/admin