Add directory feature button

This MR adds the "Add directory" feature as described in #2557 and splits out the "Create file" and "Upload file" in a dropdown.

### Screenshots

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/098b77026508eef222e0f93ec78f34ec/image.png)

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/93588b0280ffa2bd44603193feac5bdf/image.png)

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9094a63d41d7cb33e103390545c9c146/image.png)

Added branch capability to upload file form:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9b3c012a3f2333a5db0e2ceb8e6711ec/image.png)

"Upload file" is no longer accessible from the "Create file" page. Users can now select a target branch in upload file as well.
   
Closes #2799: Fixes a bug where file modes were overwritten after a commit
    
Closes https://github.com/gitlabhq/gitlabhq/issues/8253: Existing files can no longer be overwritten in the "Create file" section.
    
Closes #2557 

See merge request !1453