This change starts work on some cleanup of our legacy CSS. This starts

with the creation of three @layers:

@layer reset contains our basic rules for body padding and scroll

   behavior.
@layer defaults sets global defaults for elements (and includes our

   icon definitions so it's easy to override sizes when we want).
@layer thirdparty has rules from third party packages, including

   ink.

As part of this, I've eliminated a few global rules that were either

obsolete or just plain incorrect:


img { border: 0 } only existed because IE10 and below would add

  borders to images inside links.


h1 { font-size: 10pt } was being applied in a few situations. A

  bunch of our legacy {% box %} elements used it, mostly inside

templates/admin/manual-updates/..., the error templates, and OAuth

  authorization flow. I've added a scoped rule for .box-inner h1, and

  fixed up some layout issues, especially with the OAuth template. We

  should just replace most of this with Ink.Alert in a later cleanup.


h3 { margin-top: 2em } was visible in a few places, including the

  aforementioned manual-updates pages, as well as some of the my-account

  list views. In all of these places, it looks better without that very

  large margin. templates/admin/database.html also had bare <h2> and

<h3> elements in it, but after a trip down the rabbit hole, I've

  verified that this template is actually completely dead, so I've

  removed it.


button was applying global styles, and only conditioned on the lack

  of .ink-c-button. I've changed this to be :not([class*="-c-"]) so

  that any button that has opted into a component class won't have to

  fight against that. Legacy classes like .primary and .large still

  have these defaults applied.


Modal box buttons also had a bunch of overridden styles that were

  always applying, including digging deep into ink button groups. I've

  rewritten these to use flexboxes instead of margins.


Testing Done:

- Ran unit tests.

- Spent a bunch of time going through everything with a fine-toothed

  comb to verify that these changes were correct and did not break

  anything.

Ran unit tests.
Spent a bunch of time going through everything with a fine-toothed

  comb to verify that these changes were correct and did not break

  anything.