build(lfs): move image assets to Git LFS

Binary assets, such as JPEGs, aren't particularly compatible with Git as they inflate the repo size
if changed (since they can't be diffed). This moves them to Git LFS ahead of making changes.

It's possible to remove them from the repo entirely (i.e. remove them from previous commits) which
would make a git clone a lot faster, but since it involves rewriting history I've not gone down
that path.

Refs #392

src/frontend/assets/.gitattributes

@@ -0,0 +1,2 @@
+*.jpg filter=lfs diff=lfs merge=lfs -text
+*.png filter=lfs diff=lfs merge=lfs -text