Incubator-superset: Request access form

[mistercrunch]

When [Gamma] users do not have access to a dashboard / datasource, we currently redirect them to the welcome page and flash a message saying they don't have access.

Instead of that, we should redirect them to a place where they can request access. We can create a model and use the CRUD interface for this purpose. Fields needed for this new AccessRequest model are:

• audit information (who/when) (use the mixin!)
• dashboard(s), many to many relationship, ideally is pre-populated with where they came from
• table(s), many to many relationship, ideally is pre-populated with where they came from, based on the dashboard

Admins should be able to see the list of requests and grant access easily from the list view

[xrmx]

Can we make this optional under an option in settings please?

[bkyryliuk]

Future works:

• [ ] make it optional
• [ ] maintain a system role for people who can grant rights
• [ ] document the flows in the security section of the docs
• [ ] a quick way to see the columns in the table (tooltip? link to endpoint that list the columns for a table?)
• [ ] manage how granters could only grant permissions to a subset of dataset
• [ ] request access to the databases, not only to the datasources
• [ ] include free form message in the request form
• [ ] offer to create a role if there are 0 roles granting access to the given datasource
• [ ] email notifications
• [ ] batch approval
• [ ] store some metadata - dashboard / slide ids
• [ ] better support for the dashboards - show the slices you have access to and allow to request access to others