MDL-44826 Atto: Image alignment options

If an image has custom alignment or margins - don't overwrite
them unless a new alignment option is chosen.
MDL-44826 Atto: Set default image alignment to 'bottom'
MDL-45034 Atto: Image dialogue improvements.

1. Fix Nan bugs in auto width / height
2. Allow percentages in auto width / height
3. Change dialogue title
4. Prevent preview image resizing from changing the height of the dialogue.
5. Change wording to "Auto size"
6. Auto adjust size when the "Auto size" checkbox is toggled.
7. Add img-responsive to images with the original aspect ratio.