Improve flask

[javierdelapuente]

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flask-frameworks improvements:

• requirements.txt must contain a "flask" package.
• Error messages in flask-extension now include a link to the documentation.
• Output all messages at the same time related to missing/wrong app.py ang requirements.txt
• flask init profile:
  • Minor doc improvements.
  • Two new parts, one for runtime package slices and one for deb packages.
  • Added build-packages (pending to see if it is necessary).