In Render tolerate not being passed a context (#16842)

In Render tolerate not being passed a context (#16842)

* In Render tolerate not being passed a context

It is possible for RenderString to be passed to an external renderer if markdown
is set to be rendered by an external renderer. No context is currently sent to these
meaning that this will error out.

Fix #16835

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Add Context to Repo calls for RenderString

All calls from routers can easily add the context - so add it.

Signed-off-by: Andrew Thornton <art27@cantab.net>