Add sharing feature

[abuisman]

We are looking at replacing, amongst other things, Heroku dataclips with Blazer. The thing that we use a lot with dataclips is the sharing feature: it allows a developer to write a SQL query, they then share the CSV url with a non-technical person who can then import that into whatever tool that supports CSVs. Every time they re-download the CSV it is up-to-date with the query. When importing into Google sheets it results in an always up-to-date sheet, great all around.

In this PR I have added the feature to Blazer.

In building I realised I can still extract it and sort of patch it in, but I'd rather contribute back. I've tried to keep close to the overall design and style.

The only change I needed to make to the QueriesController that might be a good change regardless of this feature is the following:

From:

@statement = Blazer::Statement.new(params[:statement], @data_source)

To:

sql_statement = params[:statement] || @query.statement
@statement = Blazer::Statement.new(sql_statement, @data_source)

[abuisman]

I've updated the design as suggested by @bdewater. I now generate a token per query which allows for easy per-query revoking of access. Great suggestion.