Improve button outlines

[jacbn]

Reverts to using outline and outline-offset to manage button :focus rings.

I'm sure there was a reason for using box shadow at the time, but I can't find it now. Using outlines is much cleaner as it does not require that the background be a specific colour. It is less flexible r.e. shape, but for buttons this is okay (for other cases there is the focus-outline mixin that uses box-shadow when required)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.19%. Comparing base (0648363) to head (07dce2f).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2004    +/-   ##
========================================
  Coverage   43.19%   43.19%            
========================================
  Files         575      575            
  Lines       24746    24746            
  Branches     8214     8214            
========================================
  Hits        10690    10690            
- Misses      13380    14007   +627     
+ Partials      676       49   -627     

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[sjd210]

I remember you specifically making the change to migrate things over to box-shadow, but I also can't find the reason for it - and looking online there doesn't seem to be much consensus over the better way to do things.

I'm happy as long as we're mostly consistent, and I've checked over various buttons in different contexts which are working well, so this is all fine.