Create an icon font for Manage Stream checkmarks.

[showell]

We repeat the same SVG to draw checkmarks for streams in our "Manage Streams" feature. This is expensive for template rendering.

I attempted the naive fix in #10915, but browsers don't apply CSS to external SVG elements.

All the checkmarks in Manage Streams have the same shape, and same underlying SVG, but we use CSS to set the "fill" color and do things upon hover, etc.

@synicalsyntax suggested another approach here:

https://chat.zulip.org/#narrow/stream/6-frontend/subject/svg.20checkmarks/near/666289

You can use #7281 as inspiration.

[zulipbot]

Hello @zulip/server-streams members, this issue was labeled with the "area: stream settings" label, so you may want to check it out!