updates for 2018 sldl sldu

[lukemckinstry]

the purpose of this PR is to add information regarding the opencivicdata identifiers for US state legislative districts (sldu and sldl) in 2018. The PR consists of adding an updated link to the census Gazetteer files from this page on census.gov: https://www.census.gov/geographies/reference-files/time-series/geo/gazetteer-files.html

And checks in the files mapping ocd-ids to census geoids for the sldl and sldu in this folder identifiers/country-us/census_autogenerated_18/

Note: I was unable to locate the updated census url for gathering obsolete sldl and sldu for these files. So unlike the 2012 and 2014 files, these 2018 files do not list obsolete districts at the bottom of the csv.

[jdmgoogle]

After these files are updated, what's the next step in getting the country-us.csv file updated?

[lukemckinstry]

After these files are updated, what's the next step in getting the country-us.csv file updated?

For updating country-us.csv, you would run ./scripts/compile.py us. This script reads the csv files in identifiers/country-us/ and combines them into country-us.csv.
The sldl and sldu files in this PR (in identifiers/country-us/census_autogenerated_18/) are 2 of many files involved in making the us country csv but the only ones I worked with here (in order the get the census IDs for state leg districts), the census_geoid_18 column in these new files would generate a column with this name in country-us.csv when the ./scripts/compile.py us script is run.