Add a "tag icon" on the edit bar

[soliloquist-tatoeba]

Previously discussed on #785.

In order to tag sentences more easily, a tag icon on the edit bar might be very useful.

Similar to the add-to-list icon, an icon that opens a text box for adding tags when clicking on it could save a lot of time. In this way, we could tag all sentences on pages with multiple sentences without leaving that page and going to sentences' pages separately. It'll make tagging easier.