Provide a cropper for uploaded images

Description

As a user who is uploading an image to GitLab, I would like to be presented with the option to crop the image, so that I know the image will play nice with any presentation limitations imposed by CSS in GitLab.

Proposal

Mockups for the cropper

After uploading the image, if your image is larger than the limit size, you can adjust the size of image through the slider.

Links / references

N/A