Resolve "support or sqlite"

Closes #344 (closed)

Dear @carlos.vega, @jacek.lebioda

I just finished support for sqlite3 in smasch. Could you please take a look? I didn't know how to separate migration scripts between different databases. I could not even find information how to make engine dependent migration scripts in django... I would really like to hear how to improve it. It should run with sqlite 3.11 or higher.

Apart from implementing support for sqlite I also upgraded django to 2.0.13 (at some point I would like to get to the latest, but I don't have time for that right now). During the process I noticed that some django constructs are not valid anymore (like is_authenticated method now is a property, or assignment of data to ORM collections must be done with setter). Fortunately the old syntax will throw an exception when called (instead of silent faileover). However, I think we should increase code coverage - so all unsupported constructs will be caught when upgrading python dependencies.