fix(admin): use benchpress render for OAuth strategy modal

[MarionLenfant]

Summary

• On NodeBB 4, app.parseAndTranslate() returns a jQuery object, while bootbox / modals.confirm expect an HTML string for message.
• Creating or editing an OAuth2 strategy in ACP therefore fails with: "message" option must not be null or an empty string.
• Switch the New/Edit strategy modal to render() from benchpress (already used elsewhere in this file), which returns a string.

Test plan

• Install/activate this branch on NodeBB ≥ 4.14
• ACP → Plugins → Multiple OAuth2 → New Endpoint opens the strategy modal (no console error)
• Fill and save a strategy, then Edit it — modal opens with existing values
• OAuth login flow still works (runtime unchanged; ACP-only fix)

[julianlam]

render doesn't translate, hence the helper.