use icon-confirm instead of text for accepting remote share, works better with translations

Ticks another one off #8934 (server-server sharing UX review).

Currently there is a »Save« button when you click the »Add to your ownCloud« function in public sharing. This will cause problems with long strings through translations. Also it’s just ugly and doesn’t seem to belong to the input field.

This fixes that, with a confirm icon which belongs in the input field.

Please test @owncloud/designers 
@DeepDiver1975 it's a separate issue for sure. And not one to solve easily (so it's not confusing): https://github.com/owncloud/core/issues/9069