TV import files now reset old mappings

[simonreed]

Addresses #238

TV imports now remove old mappings when importing a new txt file.

[coveralls]

Coverage increased (+3.4%) to 81.468% when pulling 9bb64bb on feature/238-tv-import into 5bec349 on develop.

[spuddybike]

tests are failing - not sure if this was intended?

[simonreed]

Not intended but i'm aware. Moving this to a work in progress.

[simonreed]

I've removed the Redis records for Cluster and Strand. This means the mappings with topic conflicts are all handled by the PG database.

I've also added the UI changes so that you can now see the topic conflicts when on the dataset show page you try to :

1. Assign a question that already has a set topic which conflicts
2. Set a topic for the variable if you already have a resolved topic (from an assigned question)

When importing files for either TV or DV mappings you can now view the progress and logs for the imports. This will show you if the import has run successfully and if it fails where it failed/succeed on a line by line basis.