create custom generators

[mayel]

see https://fly.io/phoenix-files/customizing-phoenix-generators/ and https://github.com/pragdave/mix_templates and https://elixirforum.com/t/codegen-simple-succint-and-customizable-code-generation-for-your-libraries/54521

• [ ] schemas + migrations (depends on https://github.com/bonfire-networks/bonfire-app/issues/243)
• [ ] views
• [ ] components
• [x] #972
• [x] #980
• [ ] smart input type
• [ ] #981
• [ ] settings item
• [ ] new activity type + feed preview
• [ ] object type feed preview
• [ ] etc

[ivanminutillo]

Implemented a generator for creating a new extension: ´mix extension.new extension_name´ for tutorial purposes Currently it:

• clones the extension template
• renames modules name (and any appearance) according the name provided
• rename the bonfire_extension_template.exs config file (even if we do not use it anymore?)
• delete .git