Simplify migration from OSE

[vpetersson]

Migrating from Screenly OSE to Screenly is too complicated right now. it needs to be streamlined, such that the user never needs to leave the web interface (we have a CLI version already.

Here's how I would like the flow to look like:

• We add a new wizard into the web interface titled "Upgrade to Screenly"
• The first step asks the user to create an account with links and a link to the comparison guide
• The next step asks the user to enter the API key (along with info how to retrieve it)
• Once we have the API key and verify that they work, we begin the migration and show progress report