<iframe> embeds in Markdown content

Support embedding <iframes> in GitLab Flavored Markdown, enabling rich content embeds from services like YouTube, Figma, and others.

Done

• Ability to configure permitting iframe embeds in Markdown on an instance level, with an allowlist approach for domains permitted inside such an iframe.
• Embed via ![](url) syntax, where url is the iframe source.
• Allow applying width/height parameters as these embeds by default are unhelpfully small — Pass iframe dimensions through if supplied (!224624 - merged) • Asherah Connor • 18.10
• Refactor to support future work — Avoid iframes having to deal with asset proxy/l... (!224625 - merged) • Asherah Connor • 18.11
• Support transforming <iframe> as written or copy-pasted in so we can take embed code from YouTube or Figma — Treat `<iframe>` as `<img>` (!224626 - merged) • Asherah Connor • 18.11
• Support default URL transformations for YouTube/Figma, so e.g. embedding a YouTube "view" URL also Just Works™ (DWIM principle) — Transform `<iframe>` URLs according to a registry (!223870 - merged) • Asherah Connor • 19.0
• Refactor of RTE resizable embedded content — Refactor media resize into shared mixin and fix... (!231411 - merged) • Asherah Connor • 19.0
• Rich-text editor support for <iframe> embeds — Support iframe embeds in rich-text editor (!231412 - merged) • Asherah Connor • 19.0

To-do for GA

• Allow YouTube embeds to be opened in new windows — !231827
• Rework per discussion had with AppSec review — Rework <iframe> embed configuration (#599713) • Asherah Connor • 19.3
• Document (Document <iframe> embeds (#597027))

Related issues

• The original customer request for embedding GitLab pipeline artifacts in an iframe (via @jcarrano) is tracked separately in Support iframe embedding of GitLab pipeline art... (#590071).