Multiple admins for a competition

## Overview

Resolves issue #513 by adding the ability to add admins to a competition that can edit, manage and publish a competition.

## Functional review

## Tasks
- [x] Admins can't delete competition, only owner can
- [x] Admins can publish a competition
- [x] Add admins field to competitions model
- [x] Add admins to competition edit form
- [x] If someone is added as an admin, auto-add them as a participant
- [x] Admins shouldn't need secret key
- Admins can
    - [x] See competition in "My Competitions"
    - [x] Msg participants
    - [x] Manage participants
    - [x] Edit competition
    - [x] See all buttons except delete