Cells: Iteration 2[GDK] User session not shared to 2nd cell

Why we need to do this

From gitlab#415207 (closed), we expect already logged user can interact with Cell 2 without the need for re-login.

Steps to reproduce

1. Login to 1st cell: http://127.0.0.1:3000
2. Create 2nd cell using ./support/cells-add-secondary
3. Navigate to 2nd cell : http://127.0.0.1:3001

What I expect to see:

• I am logged in, and get redirect to Projects dashboard

What I see :

• Sign in page

What is going on ?

See #1894 (comment 1533712952)

Proposal

We need to fix three things:

1. Since we already in ./support/cells-add-secondary copy across db_key_base, let's do the same for secret_key_base.
2. Fix cookie key in config/initializers/session_store.rb to be the same for both GDKs
3. Update support/templates/gitlab/config/redis.sessions.yml.erb to point to GDK 1's session redis