Add PUBLIC_ROLE_LIKE_GAMMA config flag

[asydorchuk]

The flag was suggested as part of this pull request: #415. The idea behind the flag is to configure the 'Public' role with the same set of permission as the 'GAMMA' role. This simplifies configuration of dashboards visible to anonymous users, while explicit grant of 'datasource_access' is still required.

[landscape-bot]

Repository health increased by 0.05% when pulling cd8b8f0 on asydorchuk:master into 5a870fe on airbnb:master.

• 4 new problems were found (including 0 errors and 2 code smells).
• 6 problems were fixed (including 0 errors and 2 code smells).

[landscape-bot]

Repository health increased by 0.05% when pulling cd8b8f0 on asydorchuk:master into 5a870fe on airbnb:master.

• 4 new problems were found (including 0 errors and 2 code smells).
• 6 problems were fixed (including 0 errors and 2 code smells).

[landscape-bot]

Repository health decreased by 0.05% when pulling 0c9bb74 on asydorchuk:master into 5a870fe on airbnb:master.

• 2 new problems were found (including 0 errors and 0 code smells).
• No problems were fixed.

[landscape-bot]

Repository health decreased by 0.05% when pulling 0c9bb74 on asydorchuk:master into 5a870fe on airbnb:master.

• 2 new problems were found (including 0 errors and 0 code smells).
• No problems were fixed.

[mistercrunch]

Thanks for the PR. Master was broken when you branched offd, do you mind rebasing?

[landscape-bot]

Repository health decreased by 0.05% when pulling ffc53c5 on asydorchuk:master into 5c0e30e on airbnb:master.

• 2 new problems were found (including 0 errors and 0 code smells).
• No problems were fixed.

[coveralls]

Coverage increased (+0.04%) to 82.02% when pulling ffc53c5 on asydorchuk:master into 5c0e30e on airbnb:master.

[mistercrunch]

[asydorchuk]

Lol, nice gif. Thanks for the merge!