Merge commit '284b46fee7ad682b52c99d0455a1134e1d4e3fc3' into glitch-soc/merge-upstream

Conflicts:
- `app/helpers/theme_helper.rb`:
  Upstream added an experimental feature flag to try out theme refactors.
  Glitch-soc's theming system is too different, so skips the new feature flag.
- `stylelint.config.js`:
  Ditto.

app/javascript/mastodon/features/compose/components/text_icon_button.jsx

@@ -1,38 +0,0 @@
-import PropTypes from 'prop-types';
-import { PureComponent } from 'react';
-
-const iconStyle = {
-  height: null,
-  lineHeight: '27px',
-  minWidth: `${18 * 1.28571429}px`,
-};
-
-export default class TextIconButton extends PureComponent {
-
-  static propTypes = {
-    label: PropTypes.string.isRequired,
-    title: PropTypes.string,
-    active: PropTypes.bool,
-    onClick: PropTypes.func.isRequired,
-    ariaControls: PropTypes.string,
-  };
-
-  render () {
-    const { label, title, active, ariaControls } = this.props;
-
-    return (
-      <button
-        type='button'
-        title={title}
-        aria-label={title}
-        className={`text-icon-button ${active ? 'active' : ''}`}
-        aria-expanded={active}
-        onClick={this.props.onClick}
-        aria-controls={ariaControls} style={iconStyle}
-      >
-        {label}
-      </button>
-    );
-  }
-
-}