Add a dedicated "Continue" button to the terms auth stage

The button itself is conditionally enabled because the ILAG dialog already has a continue button. It'd be confusing to users to have 2 continue buttons on the same dialog, so this commit adds the structure required to pass along clicks from the dialog's button down to the UI auth component. 

The other place the continue button would appear is on the register page (not ILAG). The button's style is different here, however that will be improved in a later commit. 

Part of https://github.com/vector-im/riot-web/issues/7700

res/css/views/login/_InteractiveAuthEntryComponents.scss

@@ -40,3 +40,7 @@ limitations under the License.
     background-color: $light-fg-color;
     cursor: default;
 }
+
+.mx_InteractiveAuthEntryComponents_termsPolicy {
+    display: block;
+}