add an admin tab for managing elections

[mhl]

When someone is first setting up a new site, the first thing they need to do after it's up and running and they've looked at the settings tab is to add one or more elections. At the moment this can be done in the Django admin interface, but we'd like a better interface for this in the new easier admin (see #944)

Adding

Lots of the properties of elections can be ignored when adding a new election; you only really need to know:

• name - the name of the election (e.g. "2015 General Election")
• slug - the ID that will appear in URLs that refer to that election, e.g. "2015" (could be auto-suggested from the name, but that might generate rather long URLs)
• election_date - the date of the election
• organization - the body that successful candidates are elected to (probably with a drop-down / automatically create if a new name is typed in widget)
• party_lists_in_use - is this an election that uses party lists

Everything else can have reasonable defaults.

Editing

When editing and existing election, we would want the full list of properties of an election to be shown, but probably separating those above for the rest (which could be shown as "Advanced" options)

This might be a good opportunity to use mysociety-django-sluggable for managing the slug field and redirects if people change the election slug, as @dracos suggested here: #724 (comment)