[schema] Ensuring that the slice datasource fields are non-nullable

[john-bodley]

We've noticed a number of anomalies in our database caused by ill-defined forms and/or table schema definitions. This PR resolves a number of issues related to the slices table including:

• Ensures that the datasource_id column is not-NULL.
• Ensures that the datasource_name column is not-NULL.
• Ensures that the datasource_type column is non-NULL.

Long terms we should look at trying to deprecate the denormalized datasource_name column which can (and should) be inferred from the (datasource_id, datasource_type) tuple. I believe the reason for its existence is mostly legacy however it may be required for importing/exporting of slices where one must identify entities by name rather than ID.

Note this migration will fail if the slices.datasource_id, slices.datasource_name, or slices.datasource_type column is NULL. One must manually fix these records as programmatically trying to remedy these invalid records is difficult as it may not be apparent what the corresponding datasource should be given the possibly lack of information. The following query determines which records are problematic:

SELECT 
    * 
FROM 
    slices
WHERE 
    datasource_id IS NULL OR 
    datasource_name IS NULL OR 
    datasource_type IS NULL

Note this PR is gated by #5445 and #7084 which ensure that empty strings associated with form-data wont persist in the database and is necessary for ensuring that the relevant entries are non-NULL.

to: @graceguo-supercat @michellethomas @mistercrunch

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