emoji: Properly scale custom emoji in headings and titles

Custom emoji did have a fixed pixel based size (i.e. width & height). This makes them not properly scale in headings (h1, h2, etc.)

While at it I noticed custom emoji were not supported in titles. Because this MR is making sure proper scaling happens on headings, I'm including the support of custom emoji in titles here as well. This ensures scaling also works properly on titles.

Switch this sizing to use em's so it scales with the surrounding text.

Before	After

Issue: #381903 (closed) Label: group::project management Label: bug::functional Milestone: 16.2