Add new pin and unpin icons (#17445)

This PR adds new SVGs for the pin and unpin icons, using a custom-made
one in a smaller size, and thus removing the `PinAlt` one I added just
recently. I also tweak the outline panel pin icon button tooltip strings
a bit.

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -213,7 +213,6 @@ pub enum IconName {
     Pencil,
     Person,
     Pin,
-    PinAlt,
     Play,
     Plus,
     PocketKnife,
@@ -386,7 +385,6 @@ impl IconName {
             IconName::Pencil => "icons/pencil.svg",
             IconName::Person => "icons/person.svg",
             IconName::Pin => "icons/pin.svg",
-            IconName::PinAlt => "icons/pin_alt.svg",
             IconName::Play => "icons/play.svg",
             IconName::Plus => "icons/plus.svg",
             IconName::PocketKnife => "icons/pocket_knife.svg",