Convert cross-signing feature flag to setting

This is intended as a temporary measure until we're comfortable with removing the flag entirely.

src/components/views/dialogs/CreateRoomDialog.js

@@ -65,7 +65,7 @@ export default createReactClass({
             createOpts.creation_content = {'m.federate': false};
         }
 
-        if (!this.state.isPublic && SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (!this.state.isPublic && SettingsStore.getValue("feature_cross_signing")) {
             opts.encryption = this.state.isEncrypted;
         }
 
@@ -192,7 +192,7 @@ export default createReactClass({
         }
 
         let e2eeSection;
-        if (!this.state.isPublic && SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (!this.state.isPublic && SettingsStore.getValue("feature_cross_signing")) {
             e2eeSection = <React.Fragment>
                 <LabelledToggleSwitch label={ _t("Enable end-to-end encryption")} onChange={this.onEncryptedChange} value={this.state.isEncrypted} />
                 <p>{ _t("You can’t disable this later. Bridges & most bots won’t work yet.") }</p>

src/components/views/dialogs/DeviceVerifyDialog.js

@@ -131,7 +131,7 @@ export default class DeviceVerifyDialog extends React.Component {
                 } else {
                     this._verifier = request.verifier;
                 }
-            } else if (verifyingOwnDevice && SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+            } else if (verifyingOwnDevice && SettingsStore.getValue("feature_cross_signing")) {
                 this._request = await client.requestVerification(this.props.userId, [
                     verificationMethods.SAS,
                     SHOW_QR_CODE_METHOD,

src/components/views/dialogs/InviteDialog.js

@@ -574,7 +574,7 @@ export default class InviteDialog extends React.PureComponent {
 
         const createRoomOptions = {inlineErrors: true};
 
-        if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (SettingsStore.getValue("feature_cross_signing")) {
             // Check whether all users have uploaded device keys before.
             // If so, enable encryption in the new room.
             const client = MatrixClientPeg.get();