add cypress coverage for user text pages

loginAsSecondUser helper backs new specs that cover the
/texts list (own-only scoping, create form, link to
/texts/{id}) and /texts/{id} detail (own access, 403 on
another user's text, owner can add a child node).

cypress/support/commands.js

@@ -13,3 +13,7 @@ Cypress.Commands.add('loginAsAdmin', () => {
 Cypress.Commands.add('loginAsUser', () => {
     cy.login('user@example.com', 'password1')
 })
+
+Cypress.Commands.add('loginAsSecondUser', () => {
+    cy.login('user2@example.com', 'password2')
+})