Add ability to clear a tag value in the tag editor

Similar to (https://ideas.blisshq.com/forums/21939-bliss/suggestions/32779003-clear-tags-standard-comments-tag), when editing tags in the web-based tag editor, I should have the ability to clear a tag value (that is, remove that tag from the files). At the moment, if I clear the value for, say, Disc Number and click Save, the value is stored as "0", which then attempts to create "Disc 0" folders.