Migrations are not immutable

[flekschas]

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.