Various improvements to the signup and email confirmation process have been added to GrowthExperiments as part of T215626: [EPIC] Growth: Emailability (note that task is marked declined, and the subtasks are open, but some of what's described in them did get merged). Some of those changes are to GrowthExperiments features (the welcome survey and the homepage), but the most useful ones are the changes to the signup page and the confirmation email, and those aren't really related to GrowthExperiments and would be quite useful for all MediaWiki users (the default confirmation email is just embarrassing). We should port the code over to core.