fix: createTag now checks for spaces in tags

You mentioned in #72 that create tag page should not allow creating tags with spaces in them. While fixing that I also noticed that the dialogue lets you actually make an empty tag! And that the ui does not in any way react to receiving an error (e.g. duplicate tag). So this PR tries to fix all of the above. CreateTag mutation now checks for empty tag and tags with spaces, and the ui shows the error similar to how other ui elements (e.g. `RelativeDateTimeSelector`) do by adding a red text under the input box.