delay initialisation of id bootstrap

This fixes this issue: https://github.com/guardian/frontend/issues/6419

Easy to fix - harder to test, though I've been as thorough as I can. Identity bootstrap needs to be initialised after the discussion, otherwise there's no discussion api to retrieve the avatar url from: 

@ironsidevsquincy @chrisfinch - does this look OK to you? 

Before: 
![no-avatar](https://cloud.githubusercontent.com/assets/986155/4661903/117663b4-552a-11e4-9df7-5fdb5cd6a4c0.png)

After:
![avatar](https://cloud.githubusercontent.com/assets/986155/4661912/2458d476-552a-11e4-8df8-194ddb0c418e.png)


I'd just chuck these three in `common.js`, moving `initId` below `initDiscussion`