Closes #1242
Closes #1268

The two corrections are:

• Deleted feeds from the DB if removed form systems.csv
  • There was code that would filter out feeds that were in in the DB but not in systems.csv. Thus we did not know that some feeds were removed from systems.csv
• Stopped processing feeds based on changes in location
  • The location is initially set with the populate scripts from systems.csv, but then it is changed by the function that determines the location from the data. So comparing locations to see if something changed in CSV did not work.

For deletion, this PR also setup cascade deletes on certain DB tables in the SQL schema. For example, id a GbfsFeed is deleted, the entry in gbfsversion table referring to this feed is also deleted. This prevent failing of the deletion because gbfsversion.feed_id cannot be null and is a foreign key to gbfsfeed.

Testing tips:
See test_populate_gbfs.py for the tests that have been done.
We miodified the way the DB is filled in the api integration tests to allow 2 .csv files to be loaded. This allowed to test loading a csv in a DB that is already populated, as was necessary for this PR.

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