Use an icon instead of "Link" text to copy link to uploaded file

[nqst]

I think "Link" is not the best text for the button which performs an action (we should use verbs; but just "Copy" is incorrect and "Copy link" is too long). Also, it feels redundant when there are a lot of files:

I propose to use an icon instead, like this one:

Note: Copy icon might not work perfectly if we keep the uploading method icons (see #1412)

[kvz]

This issue will be closed when we merge the PR 🎉