[PLAY-1516] Make Rails icons fixed-width (#3656)

**What does this PR do?** A clear and concise description with your
runway ticket url.
https://runway.powerhrg.com/backlog_items/PLAY-1516

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3. Scroll down to '....'
4. See addition/change


#### Checklist:
- [ ] **LABELS** Add a label: `enhancement`, `bug`, `improvement`, `new
kit`, `deprecated`, or `breaking`. See [Changelog &
Labels](https://github.com/powerhome/playbook/wiki/Changelog-&-Labels)
for details.
- [ ] **DEPLOY** I have added the `milano` label to show I'm ready for a
review.
- [ ] **TESTS** I have added test coverage to my code.

---------

Co-authored-by: Gary Kang <[email protected]>

playbook/app/pb_kits/playbook/pb_icon/icon.rb

@@ -9,7 +9,7 @@ class Icon < Playbook::KitBase
       prop :border, type: Playbook::Props::Boolean,
                     default: false
       prop :fixed_width, type: Playbook::Props::Boolean,
-                         default: false
+                         default: true
       prop :flip, type: Playbook::Props::Enum,
                   values: ["horizontal", "vertical", "both", nil],
                   default: nil

playbook/spec/pb_kits/playbook/kits/icon_spec.rb

@@ -75,18 +75,18 @@
       rotation = 90
       size = "sm"
 
-      expect(subject.new(icon: icon, border: true).classname).to eq "pb_icon_kit far fa-user fa-border"
-      expect(subject.new(icon: icon, fixed_width: true).classname).to eq "pb_icon_kit far fa-user fa-fw"
-      expect(subject.new(icon: icon, flip: "horizontal").classname).to eq "pb_icon_kit far fa-user fa-flip-horizontal"
-      expect(subject.new(icon: icon).classname).to eq "pb_icon_kit far fa-user"
-      expect(subject.new(icon: icon, inverse: true).classname).to eq "pb_icon_kit far fa-user fa-inverse"
-      expect(subject.new(icon: icon, list_item: true).classname).to eq "pb_icon_kit far fa-user fa-li"
-      expect(subject.new(icon: icon, pull: pull).classname).to eq "pb_icon_kit far fa-user fa-pull-#{pull}"
-      expect(subject.new(icon: icon, pulse: true).classname).to eq "pb_icon_kit far fa-user fa-pulse"
-      expect(subject.new(icon: icon, rotation: rotation).classname).to eq "pb_icon_kit far fa-user fa-rotate-#{rotation}"
-      expect(subject.new(icon: icon, size: size).classname).to eq "pb_icon_kit far fa-user fa-#{size}"
-      expect(subject.new(icon: icon, spin: true).classname).to eq "pb_icon_kit far fa-user fa-spin"
-      expect(subject.new(icon: icon, classname: "additional_class").classname).to eq "pb_icon_kit far fa-user additional_class"
+      expect(subject.new(icon: icon, border: true, fixed_width: false).classname).to eq "pb_icon_kit far fa-user fa-border"
+      expect(subject.new(icon: icon).classname).to eq "pb_icon_kit far fa-user fa-fw"
+      expect(subject.new(icon: icon, flip: "horizontal").classname).to eq "pb_icon_kit far fa-user fa-fw fa-flip-horizontal"
+      expect(subject.new(icon: icon).classname).to eq "pb_icon_kit far fa-user fa-fw"
+      expect(subject.new(icon: icon, inverse: true).classname).to eq "pb_icon_kit far fa-user fa-fw fa-inverse"
+      expect(subject.new(icon: icon, list_item: true).classname).to eq "pb_icon_kit far fa-user fa-fw fa-li"
+      expect(subject.new(icon: icon, pull: pull).classname).to eq "pb_icon_kit far fa-user fa-fw fa-pull-#{pull}"
+      expect(subject.new(icon: icon, pulse: true).classname).to eq "pb_icon_kit far fa-user fa-fw fa-pulse"
+      expect(subject.new(icon: icon, rotation: rotation).classname).to eq "pb_icon_kit far fa-user fa-fw fa-rotate-#{rotation}"
+      expect(subject.new(icon: icon, size: size).classname).to eq "pb_icon_kit far fa-user fa-fw fa-#{size}"
+      expect(subject.new(icon: icon, spin: true).classname).to eq "pb_icon_kit far fa-user fa-fw fa-spin"
+      expect(subject.new(icon: icon, classname: "additional_class").classname).to eq "pb_icon_kit far fa-user fa-fw additional_class"
     end
     it "includes color class when color prop is provided", :aggregate_failures do
       icon = "user"