This is the first step in cleaning up our CSS rules, with the aim of

bringing some consistency, organization, and to make some rules more

reusable for extensions.
defs.less's mixins have been moved out into a set of files in mixins/.

defs.less pulls those in, meaning that any existing consumer of

defs.less will continue to work.
common.less has too much junk in it. Some of the things in there are

reusable UI components, like boxes, buttons, and banners. Those three

have been pulled out into files in a ui/ directory.
My goal is to get all the styles into consistent places over the next

few changes, and then begin to standardize some of the naming, hopefully

coming up with a standard set of classes that we can document and reuse

in extensions.

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