Use 'Sign In' / 'Sign Out' universally

Rather than a mix of 'sign in/out' and 'log in/out'

src/components/views/login/CustomServerDialog.js

@@ -27,7 +27,7 @@ module.exports = React.createClass({
                 </div>
                 <div className="mx_Dialog_content">
                     <span>
-                        You can use the custom server options to log into other Matrix
+                        You can use the custom server options to sign into other Matrix
                         servers by specifying a different Home server URL.
                         <br/>
                         This allows you to use this app with an existing Matrix account on

src/components/views/login/PasswordLogin.js

@@ -104,7 +104,7 @@ module.exports = React.createClass({displayName: 'PasswordLogin',
                     placeholder="Password" />
                 <br />
                 {forgotPasswordJsx}
-                <input className="mx_Login_submit" type="submit" value="Log in" />
+                <input className="mx_Login_submit" type="submit" value="Sign in" />
                 </form>
             </div>
         );