Enable first mini-migration for the kit (not for plugins)

[nataliecarey]

What

We're aware that some users who have been through a migration to v13 are missing app/views/layouts/main.html, we should add a feature where we detect users in this state and offer to resolve it for them.

This is the first "mini migration" we're planning to offer, there will be other similar tasks further down the line.

Later on we may wish to add this as functionality plugin developers can use to help users upgrade when they make breaking changes but that's out of scope for this ticket.

Users should be presented with a button they can press to perform this migration, it shouldn't be done for them without their consent. We should also discuss:

• Undo functionality (maybe just making sure they've done a git commit before they press the button)
• Hiding the migration if they don't want to do it

The implementation should detect users in this state and offer to fix it for them.

Why

In v13 we moved away from editing user's files for them, there are times when we can offer them help by updating their files for them.

Who needs to work on this

• Designer
• Developer

Who needs to review this

• Designer
• Developer

Done when

• There is a button the user can click to create an app/views/layouts/main.html and keep their pages behaving the same as before (e.g. moving app/views/layout.html and then rewriting the relevant `{% extends %} statements.