uk-dropdown-scrollable height fix

- Dropdown should have a max-height, not implicit height set in the event there are not enough dropdown items to warrant the overflow.
Merge pull request #540 from zslabs/dropdown-scrollable

uk-dropdown-scrollable height fix
Fixes #498
cleanup
Fixes #451
Added iframe option + load custom css in iframe (htmeditor add-on)
missing htmleditor.less updates for iframe mode
changelog updated
codemirror updated to 4.4
codemirror options updated
Fixes #553