Create a simple CRUD app with clean modern styling

[jonasjabari]

We should show how a simple CRUD data flow would be handled with matestack. We will deploy this repository (will be a different issue) and link to it from matestacks documentation. As we don't want first time visitors to think that matestack can't look good, we should apply clean modern ("good looking") styling on top of the CRUD flow.

• The CRUD app should live right next to other matestack apps in this repository e.g. app/matestack/apps/crud_example.rb
• Routes and controller should be scoped with a name mapped to the matestack app name
• We might use the classic example of a TODO-List (just a proposal --> if you have something better in mind, go for it).
• Therefore we need the relevant Active Record Model(s) (Task, if something more complex should be shown we could add Assignee)
• We could use materialize for styling in order to make it look good and modern easily
• The source code should be readable and probably documented in order to give early matestack users a good orientation on how to develop things. Don't make it too complex even if you are able to make it fancy with some ruby language features
• Feel free to adjust requirements! I think the intention is clear and could be summarized as

--> Onboard early matestack users with a practical example
--> Deploy a tangible example for visitors and convince them that matestack could be used to create something cool and good looking very easy with few and maintainable code

Note: Just regard this issue as starting point; we gonna extend the example applications step by step using further issues and proposals then and will add new components from the core as well

[jonasjabari]

@PasWen Did I forget anything?