Discourse New Topic button theme component

A simple theme to add a "New Topic" button on every page.

Button text

• show_button_text — show a text label next to the icon. Disable it for an icon-only button.
• new_topic_button_text — custom label. Leave it empty (the default) to use core's own topic.create string, which is translated into each visitor's language.
• new_topic_button_title — tooltip; falls back to the label when empty.

Sites upgrading from before show_button_text existed are migrated automatically: an empty label becomes show_button_text: false, and the old default "New Topic" becomes empty so those sites pick up core's translations.

More information: https://meta.discourse.org/t/new-topic-button-on-all-pages-theme-component/84551