fix isCreatableName and test

lib/models/page.js

@@ -395,6 +395,7 @@ module.exports = function(crowi) {
       /\^|\$|\*|\+|#/,
       /^\/-\/.*/,
       /^\/_r\/.*/,
+      /^\/_apix?(\/.*)?/,
       /^\/?https?:\/\/.+$/, // avoid miss in renaming
       /\/{2,}/,             // avoid miss in renaming
       /\s+\/\s+/,           // avoid miss in renaming

test/models/page.test.js

@@ -200,7 +200,9 @@ describe('Page', () => {
       expect(Page.isCreatableName('/meeting/edit')).to.be.false;
 
       // under score
-      expect(Page.isCreatableName('/_')).to.be.false;
+      expect(Page.isCreatableName('/_')).to.be.true;
+      expect(Page.isCreatableName('/_template')).to.be.true;
+      expect(Page.isCreatableName('/__template')).to.be.true;
       expect(Page.isCreatableName('/_r/x')).to.be.false;
       expect(Page.isCreatableName('/_api')).to.be.false;
       expect(Page.isCreatableName('/_apix')).to.be.false;