icons: Add checked_circle_{,un}checked from Figma, with modifications

(The _checked/_unchecked suffix is added; they're both called
"checked_circle" in Figma.)

When I use the SVGs from Figma without modifications, these look
wrong; seems like the filled/unfilled areas are inverted, and the
background is a solid square instead of transparent. Figma link:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=62-8121&m=dev

So I deleted some goo that looks like it's not needed anyway. Here
are the diffs:

```
@@ -1,10 +1,3 @@
 <svg width="25" height="24" viewBox="0 0 25 24" fill="none" xmlns="http://www.w3.org/2000/svg">
-<g clip-path="url(#clip0_62_8120)">
 <path d="M12.5 2C6.98 2 2.5 6.48 2.5 12C2.5 17.52 6.98 22 12.5 22C18.02 22 22.5 17.52 22.5 12C22.5 6.48 18.02 2 12.5 2ZM12.5 20C8.09 20 4.5 16.41 4.5 12C4.5 7.59 8.09 4 12.5 4C16.91 4 20.5 7.59 20.5 12C20.5 16.41 16.91 20 12.5 20Z" fill="black"/>
-</g>
-<defs>
-<clipPath id="clip0_62_8120">
-<rect width="24" height="24" fill="white" transform="translate(0.5)"/>
-</clipPath>
-</defs>
 </svg>
```

```
@@ -1,10 +1,3 @@
 <svg width="25" height="24" viewBox="0 0 25 24" fill="none" xmlns="http://www.w3.org/2000/svg">
-<g clip-path="url(#clip0_62_8121)">
 <path d="M12.5 2C6.98 2 2.5 6.48 2.5 12C2.5 17.52 6.98 22 12.5 22C18.02 22 22.5 17.52 22.5 12C22.5 6.48 18.02 2 12.5 2ZM9.79 16.29L6.2 12.7C5.81 12.31 5.81 11.68 6.2 11.29C6.59 10.9 7.22 10.9 7.61 11.29L10.5 14.17L17.38 7.29C17.77 6.9 18.4 6.9 18.79 7.29C19.18 7.68 19.18 8.31 18.79 8.7L11.2 16.29C10.82 16.68 10.18 16.68 9.79 16.29Z" fill="black"/>
-</g>
-<defs>
-<clipPath id="clip0_62_8121">
-<rect width="24" height="24" fill="white" transform="translate(0.5)"/>
-</clipPath>
-</defs>
 </svg>
```

Author: chrisbobbe