[pkvach]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

Adds DOM events so embedding pages can react to rendered comment markup without overriding Isso's stylesheet.

• Emit isso-postbox-rendered, isso-comment-rendered and isso-thread-rendered DOM events.
• Add isso-postbox-submit, isso-postbox-preview and isso-postbox-edit classes to postbox buttons for easier restyling.

Example:

document.addEventListener('isso-postbox-rendered', function (e) {
    e.detail.element.querySelector(".isso-postbox-submit")
        .classList.add('btn', 'btn-primary');
});

Why is this necessary?

Closes #1003