Fronts components in admin

Adds a new Front section to the Football Admin. This allows editors to select components for inclusion in the Facia Tool.

Also tidies the football admin by removing some of the hard-coded information, in particular the all-important hard-coded season dates.

Minor changes to sport were required to for this change, specifically allowing the competition's id in place of its tag for the JSON calls that power fronts. The changes are otherwise confined to the football admin. Integration with Facia is ongoing, this PR supports that process.

![football-fronts-admin](https://cloud.githubusercontent.com/assets/29761/2831355/324fe3e6-cfb8-11e3-8c75-9950d19cd40c.png)