Add 'Start new season' functionality to admin

[Kechr]

Admins should be able to start a new season from the admin page.

The admin should be able to

• [x] Create a new season
• [x] Edit an existing season
• [x] Delete an existing season

[Faustvii]

Sounds like we have two places we would like a date component of some kind.

• Creating/editing seasons
• Editing matches

[Kechr]

@Faustvii The native datetime picker seems good enough IMO