Nominator can't join its pool in Pools Overview page

[frankli-dev]

Here are my steps.

1. There are two validators Alice & Bob
2. Using Charlie, I create a pool AT 01 (selecting Alice & Bob as validators)
3. When creating a pool, I select Ferdie as a nominator.
4. After pool is created, if I select Ferdie account, the UI doesn't show any Join button as below

I think we should support Nominator can also join its pool on this page (pools/overview).

[rossbulat]

An upcoming nomination pool migration will mean that nominators will no longer be able to become a pool member. This will affect the dashboard UI - perhaps this issue is better suited to monitor this change instead.

The dashboard will need to make it obvious that it is not possible to join a pool or existing nominators. This could just entail a prompt on the Pools page along with disabled buttons.