Reorganize CSS files (#24739)

Reorganize various CSS files for clarity, group together by subdirectory
in `index.css`. This reorders some of the rules, but I don't think it
should introduce any issues because of that.

web_src/css/editor/fileeditor.css

@@ -0,0 +1,87 @@
+.repository.file.editor .tab[data-tab="write"] {
+  padding: 0 !important;
+}
+
+.repository.file.editor .tab[data-tab="write"] .editor-toolbar {
+  border: 0 !important;
+}
+
+.repository.file.editor .tab[data-tab="write"] .CodeMirror {
+  border-left: 0;
+  border-right: 0;
+  border-bottom: 0;
+}
+
+.repo-editor-header {
+  display: flex;
+  margin: 1rem 0;
+  padding: 3px 0;
+}
+
+.editor-toolbar {
+  border-color: var(--color-secondary);
+}
+
+.editor-toolbar.fullscreen {
+  background: var(--color-body);
+}
+
+.editor-toolbar button {
+  border: none !important;
+  color: var(--color-text-light);
+}
+
+.editor-toolbar button:not(:hover) {
+  background-color: transparent !important;
+}
+
+.editor-toolbar i.separator {
+  border-left: none;
+  border-right-color: var(--color-secondary);
+}
+
+.editor-toolbar button:hover {
+  background: var(--color-hover);
+}
+
+.editor-toolbar button.active {
+  background: var(--color-active);
+}
+
+/* hide preview button, we have the preview tab for this */
+.editor-toolbar:not(.fullscreen) .preview {
+  display: none;
+}
+
+/* hide revert button in fullscreen, it breaks the page */
+.editor-toolbar.fullscreen .revert-to-textarea {
+  display: none;
+}
+
+.editor-preview {
+  background-color: var(--color-body);
+}
+
+.editor-preview-side {
+  border-color: var(--color-secondary);
+}
+
+.editor-statusbar {
+  color: var(--color-text-light);
+}
+
+.editor-loading {
+  padding: 1rem;
+  text-align: center;
+}
+
+.edit-diff {
+  padding: 0 !important;
+}
+
+.edit-diff > div > .ui.table {
+  border-top: none !important;
+  border-bottom: none !important;
+  border-left: 1px solid var(--color-secondary) !important;
+  border-right: 1px solid var(--color-secondary) !important;
+}