Workaround of bug 72960, disabling WikiEditor dialog in Page namespace

Workaround of issue when dialog class of WikiEditor isn't being loaded,
affecting both WikiEditor functinality and tools/user scripts depending on that class.

Some more long term solution would be to fix this in WikiEditor, by treating CONTENT_MODEL_PROOFREAD_PAGE same way as CONTENT_MODEL_WIKITEXT

Bug: 72960
Bug: T72960
Change-Id: I6830b97519ed6027c198552eeaed86b82eb9ea30