[Core] Removes utils/extend.js

[newoga]

This commit removes the extend.js utility. We already have enough existing merge alternatives such as our own style merger, lodash.merge, react-addon-update, Object.assign, and ES6 spread operators. I replaced each use of utils/extend with the "most appropriate" alternative according to code consistency and then removed utils/extend.js.

Note: Technically extend.js is a recursive merger. lodash.merge is also a recursive merger. Based on use cases, it seems like styles/theme-manager is the only case that actually requires recursive merging.

[oliviertassinari]

@newoga Yeah! Let's get rid of this technical dept 🎉.

[mbrookes]

"debt". Getting rid of the technical department is probably not what you want :winking:

[alitaheri]

@newoga Thanks 😁

[oliviertassinari]

@mbrookes I would rather be a native speaker 😁

[igorbt]

@newoga @oliviertassinari are you sure that this didn't break anything? Removed extend util was always returning a new object, without mutating the base (https://github.com/callemall/material-ui/blob/v0.14.2/src/utils/extend.js). lodash.merge is actually mutating the base (https://lodash.com/docs#merge).

[newoga]

@igorbt This code has gone through a few refactorings (for example neither Extend and mergeStyles() in this PR is being used in master at the moment). Currently Object.assign is being used and it is creating new objects when necessary. Could you open a new issue and describe the problem you're having in detail and what version you're having it in?