Using a submodule for bender theme

The idea is pretty simple: use a submodule for bender theme.

## Why?

Now we are not sure where we have to develop bender theme. Is it in the osclass-themes repository? Or it's in the osclass main repo? IMHO, I go with developing all the themes in osclass-themes.

## Pros

The theme has his own repo and it's not mix up with the core timeline of changes.

## Cons

The submodule is just a reference to a commit hash. So, once in a while in addition to doing commits in the bender repository we should update the submodule so the reference is to the correct place.

## Conclusion

This is just an idea. What I'm 100% sure it's that we should chose one: develop the theme in the core repo or in the theme repo. I don't think that it's a good idea copying the files from one repo to the other.

> **Note**: I'm not 100% sure that it's the way to go. However, it's how I think the submodules are used.