Update flask postgres tutorial with clarifications and more information

[nnja]

Made the following changes to update the tutorial and clarify some of the steps:

• Cleaned up extraneous whitespace
• Fixed the command to connect to postgres locally
• Added a note about a common issue on postgres installs on osx
• Removed duplicate 'try it' section for cloud shell
• Replaced the sku-name value on the Postgres instance creation with one of a lower price tier, and added an explanation of what the sku-name flag is used for.
• Cleaned up the firewall rules section, fixed a mistaken description, and added extra clarification around the rule for allowing Azure IPs
• Added a section to the end to walk through cleaning up resources created in this tutorial.

Includes clean up for the sample code, via Azure-Samples/docker-flask-postgres#3

[PRMerger13]

@nnja : Thanks for your contribution! The author, @berndverst, has been notified to review your proposed change.

[nnja]

CC @cephalin

[ktoliver]

@nnja

Microsoft authors of articles need to make updates through the private repository so the updates can be staged and validated by the current validation rules. When you work in the private repo, after you review staging and fix all validation issues, you can add the sign-off comment to let the reviewers know the updates are ready to be merged. Read about article authors working in the public repo for more information.

Would you close this pull request and submit the content in a pull request in the private repo, so automation can take the corresponding action? Thank you.

[cephalin]

@nnja Actually also we're in the process of updating the tutorial to target the Python built-in image. Let's merge this change into that work because there will be quite a bit of structural changes to the sample repo.

[nnja]

Hi @ktoliver. Thanks for the info! Since @cephalin has in flight changes to this doc, he requested that I leave this PR as is for now. I'll make future updates in the private repo.

[cephalin]

@nnja please close this now in lieu of the other PR