[FEATURE] upload ui

__Tasks:__

- [x] create ui for media upload
- [x] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

Includes this branch: https://github.com/sulu-cmf/SuluMediaBundle/pull/3

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none