Clarify that device names are publicly visible

And also a bunch of other UI fixes in the devices table:
 * It's the devices table, don't need 'device' in all the headers
 * Not really necessary to label checkboxes with 'select'
 * Stop table from moving down when the delete button appears

Fixes https://github.com/vector-im/riot-web/issues/10216

src/components/views/settings/tabs/user/SecurityUserSettingsTab.js

@@ -258,6 +258,7 @@ export default class SecurityUserSettingsTab extends React.Component {
                 <div className="mx_SettingsTab_section">
                     <span className="mx_SettingsTab_subheading">{_t("Devices")}</span>
                     <div className='mx_SettingsTab_subsectionText'>
+                        {_t("A device's public name is visible to people you communicate with")}
                         <DevicesPanel />
                     </div>
                 </div>