Move insert component button "visually-hidden" mark up into the theme template

Problem/Motivation

The hidden "Choose component" text in the button has been raised as a usability issue on a project I'm working on. This feels like a theme-level decision and it would be more theme-friendly if this markup lived in the template rather than the render array.

Proposed resolution

Move markup into layout-paragraphs-insert-component-btn.html.twig

Remaining tasks

* Agree on proposed resolution
* Write patch

User interface changes

None

API changes

Could potentially change sites that already override this template.