Avoid visual glitch when terms appear for IM

This avoids a visual glitch where the Integration Manager portal would briefly
appear, but then be replaced by a smaller Terms dialog when there's something to
agree to.

To resolve this minimal code churn, this cheats a bit and customises the size of
the terms dialog to match the IM portal modal when terms are shown for IM
purposes.

Fixes https://github.com/vector-im/riot-web/issues/10386

src/components/views/dialogs/TermsDialog.js

@@ -176,7 +176,7 @@ export default class TermsDialog extends React.PureComponent {
         }
 
         return (
-            <BaseDialog className='mx_TermsDialog'
+            <BaseDialog
                 fixedWidth={false}
                 onFinished={this._onCancelClick}
                 title={_t("Terms of Service")}