Migrations are not immutable

Related to #49. I checked the models and migration and notices that the ViewConf tables should have been created but Django kept telling me that all migrations have already been applied. The I checked the history of the migrations (https://github.com/hms-dbmi/higlass-server/blob/master/tilesets/migrations/0001_initial.py) and realized that they are not immutable. This is tricky as it makes updating existing server instances really painful.

Migrations should be immutable and instead new migrations should be added.

Start by comparing the models with tilesets/migrations/0001_initial.py and review Django's migration history. Identify schema changes present in the models but absent from the applied migration, then add them through a new migration rather than changing 0001_initial.py. Done means existing server instances can detect and apply the missing migration.

Written by the indexing model from the issue text.