Enable "Allow Dangerous Changes" config for Mercurial repositories

Summary:
Fixes T4357. The Mercurial commit hooks enforce dangerous change protection, but the UI doesn't actually let you configure it.

This was just an oversight (I never went back and enabled it) -- allow it to be configured in the UI.

Test Plan: Clicked "Edit Repository" on a hosted Mercurial repository, saw option to enable dangerous changes.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4357

Differential Revision: https://secure.phabricator.com/D8108