Discussion "Switch type" option

[linc]

So far we have Q&A and Ideation that both introduce new Types of discussions. There is a discrete process for switching types, and new options are needed in the discussion menu to allow for it.

My proposal is a "Switch type" or "Convert" option that:

• Has a basic form popup view that 1 or more plugins can hook into.
• Fires a generic "Convert" event that plugins can hook into.

This will de-duplicate similar code in addons and give us a clean way of extending this feature as we increase the number of plugins relying on it.

[linc]

Supplanted by #5387