Frontpage changes

This PR removes the `FrontPage.fronts` `Seq` and replaces it with a more programmatic solution to deal with custom fronts.

There are now two things you can set in the tool for a front: `keyword` and `webTitle`.

`keyword` is required; if it is not set, then `webTitle` will be ignored. The reason this is a requirement is for generation purposes. We can't generate anything useful from a `webTitle`, therefore require a `keyword` to generate different values for a page.

@stephanfowler 

@ironsidevsquincy @phamann Are there any javascript dependencies on any of these fields at the moment on fronts?

### Example
`keyword: Greenland`, `webTitle: Test`

---
```
id: greenland
section: greenland
analyticsName: GFE:greenland
description: Latest Greenland news, comment and analysis from the Guardian, the world’s leading liberal voice
webTitle: Test
```

---

### Example 2
`keyword: Greenland` (No webTitle)

---
```
id: greenland
section: greenland
analyticsName: GFE:greenland
description: Latest Greenland news, comment and analysis from the Guardian, the world’s leading liberal voice
webTitle: Latest Greenland news, comment and analysis from the Guardian, the world’s leading liberal voice
```

---

### Default
```
id: ""
section: ""
analyticsName: "GFE:Network Front"
description: "Latest news, comment and analysis from the Guardian, the world’s leading liberal voice"
webTitle: String: "Latest news, sport and comment from the Guardian"
```