Add explanation for disabled share buttons

There is currently no explanation why a share button is disabled:


There can be several reasons for a share button to be disabled. (unsaved search, search/dashboard not owned by the user)

One approach could be to update the `title` attribute to explain why the button is disabled. @linuspahl had another suggestion:

> In general: We talked about displaying an info icon next to the text which explains in a popover that you need to be owner to e.g. share a dashboard. That is one possible solution. I prefer it over a read-only icon.