It is short for "That is you done". It signifies that Madam Malkin has finished putting pins into robe that Harry is wearing. It is a common (UK) English phrasing used to mean that someone has finished the work, or a stage of the work, that they are doing on behalf of another person. Here the pins have been placed to show the wanted length and next someone can sew the robe to that length.
The whole ending of that paragraph is written in a poetic register, violating the rules of grammatical prose and even normal usage. There is no rule against trying to write in a poetic register except that others may find it obscure, pretentious, or both.
The force that through the green fuse drives the flower drives my red blood is meaningless if interpreted literally, but it is great poetry. Of course, few of us can match Dylan Thomas, who wrote the only decent villanelle in the English language, so, in my view, few of us should even try.
The meaning of that purple prose is What was needed -- what Emily needed -- was a woman emancipated from the pretences of tradition, a woman as strong as any father imagined by tradition, a woman willing to scorn the dictates of traditional deference, willing to speak out and to act.
Some can write like Dylan Thomas; most of us cannot. In my opinion, those of us who cannot should learn to write prose. As written, the quoted passage is facially absurd: at what time in the history of the world were women not able to speak. What is meant is that women would be punished, socially or physically, for doing certain things and so, quite rationally, were frequently unwilling to do them. The women who emancipated women were brave women who did what other women could have done but feared to do. I wear my clothes, not men's clothes.
EDIT: The quote about clothes (actually a slight mis-quote) is from the 19th century feminist doctor, Mary Edwards Walker, the only woman to have won the Cobgressional Medal of Honor, which was awarded for her work as a battlefield surgeon during the Civil War. I should have attributed it initially.
Best Answer
I am a non-enforcement employee of the NYPD.
The rules, regulations, and procedures of the Department are compiled in two books, called the "Patrol Guide" and the "Administrative Guide". Because of court decisions, changes in the law, and so on, changes in the rules/regulations/procedures occasionally must be made. When a change is made, a "General Order" is issued, documenting the change. The General Order remains in effect as such until the revised rule/regulation/procedure is incorporated into the next Guide update (which usually happens once per year). However, it is often the case that a procedure becomes known by its General Order number, even years after it has been incorporated into the Guide. This is the case with General Order 16, in the show - by now, it has been incorporated into the Guide, but it became so well-known as a General Order that the original name has 'stuck'. (Similar things happen to standardized paperwork; it has been decades since a complaint report [report of a crime] has borne the form ID number "61", but you still hear them called "sixty-ones".)
I can guess at what current procedure is being referred to, but it's not actually relevant for the purposes of this question. The phrase "You're being G.O. 16'd" simply means that the Department is invoking the provisions of the Guide procedure originally promulgated as General Order 16, and the person being spoken to in the scene is the target. The subsequent statements imply that the procedure in question is a disciplinary investigation, and if the target of the investigation is caught lying, it will be sufficient grounds, in and of itself, to get the target fired.