Adding the files would help

res/css/views/settings/_E2eAdvancedPanel.scss

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+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_E2eAdvancedPanel_settingLongDescription {
+    margin-right: 150px;
+}
+

src/components/views/settings/E2eAdvancedPanel.js

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+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from 'react';
+
+import * as sdk from '../../../index';
+import {_t} from "../../../languageHandler";
+import {SettingLevel} from "../../../settings/SettingsStore";
+
+const SETTING_MANUALLY_VERIFY_ALL_SESSIONS = "e2ee.manuallyVerifyAllSessions";
+
+const E2eAdvancedPanel = props => {
+    const SettingsFlag = sdk.getComponent('views.elements.SettingsFlag');
+    return <div className="mx_SettingsTab_section">
+        <span className="mx_SettingsTab_subheading">{_t("Advanced")}</span>
+
+        <SettingsFlag name={SETTING_MANUALLY_VERIFY_ALL_SESSIONS}
+            level={SettingLevel.DEVICE}
+        />
+        <div className="mx_E2eAdvancedPanel_settingLongDescription">{_t(
+            "Individually verify each session used by a user to mark it as trusted, not trusting cross-signed devices.",
+        )}</div>
+    </div>;
+};
+
+export default E2eAdvancedPanel;

src/settings/controllers/PushToMatrixClientController.js

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+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { MatrixClientPeg } from '../../MatrixClientPeg';
+
+/**
+ * When the value changes, call a setter function on the matrix client with the new value
+ */
+export default class PushToMatrixClientController {
+    constructor(setter, inverse) {
+        this._setter = setter;
+        this._inverse = inverse;
+    }
+
+    getValueOverride(level, roomId, calculatedValue, calculatedAtLevel) {
+        return null; // no override
+    }
+
+    onChange(level, roomId, newValue) {
+        // XXX does this work? This surely isn't necessarily the effective value,
+        // but it's what NotificationsEnabledController does...
+        this._setter.call(MatrixClientPeg.get(), this._inverse ? !newValue : newValue);
+    }
+}