create sameAs relationship for detroit city, MI wards

[nuttingd1]

In the current Detroit charter, 'districts' and 'wards' are used interchangeably to describe the drawn political units for which the city council, board of police commissioners, and community advisory councils are elected (§2-105.A.12 and §3-108), and I think they should be aliased in the repository. I'd like to make the council_district types the preferred identifier, since the city council refers to their political geographies as 'districts' on their official website, interactive map, and census data map ('districts' are similarly used on the board of police commissioners and community advisory council websites).

Additionally, there are only 7 electoral districts from which members of these bodies are elected (save for the at-large members), so I added validThrough dates for wards 8-22 currently in the repository for this year. It doesn't look like there have been more than 7 electoral districts in the recent past, and I can't find any instances where these additional districts are currently in use. Not sure what the right move is here, but I could see removing them from the repository altogether if that makes more sense.

TLDR, here are the changes I made: -create mi-aliases file -move 'ward' identifiers to mi-aliases file, add validThrough dates for wards no longer in use -make 'council_district' the preferred identifier by adding sameAs fields for wards 1-7

[nuttingd1]

@jloutsenhizer @jpmckinney just tagging y'all for review, thanks!

[jpmckinney]

@nuttingd1 Can you investigate why the diff for identifiers/country-us/state-mi-local_gov.csv is the entire file? Did the line endings change? Preferably, the commit would be substituted for one that produces a smaller diff.

Also, this branch conflicts, so that needs to be resolved.

[nuttingd1]

submitted a new pull request to address the diff issues in #356