Add "swap" button for teams in fixtures view

[eurochriskelly]

Description

At the push of a button,

• select team 1 (highlight all same)
• select team 2 (highlight all same)
• swap both teams across groups if necessary

This is often required during planning or even on the day in the morning. It can also be changed based on a fairness basis or on a team availability basis (e.g. due to delayed flights)