Provide a way to update existing links

[matthiaskern]

Instead of only showing the date of the last link revision, we should show a button to update a link to the latest revision.

#190

[kyldvs]

Being able to update them easily would be great, the two features I'd want are:

1. Add a "refresh" button that will update to the latest version when clicked
2. When adding the link, add an option that can be chosen to always use the latest version of linked sketch (probably harder to implement)