Bug 1047521 - welcome.wmo redirects

## To test in browser:

### mentors page
1. <http://localhost:9006/signup/mentors>
2. Sign up with a new webmaker account, or log in with persona

##### expected result
page redirects to <http://localhost:9006/for/mentors>

### learners page
1. <http://localhost:9006/signup/learners>
2. Sign up with a new webmaker account, or log in with persona

##### expected result
page redirects to <http://localhost:9006/for/learners>