Add flags for bv, hm, sj, and um, per #21

* `bv` Bouvet Island → `no` Norway
* `hm` Heard Island and McDonald Islands → `au` Australia
* `sj` Svalbard and Jan Mayen → `no` Norway
* `um` United States Minor Outlying Islands → `us` United States of America

flags/bv.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><mask id="a"><circle cx="256" cy="256" r="256" fill="#fff"/></mask><g mask="url(#a)"><path fill="#d80027" d="M0 0h100.2l66.1 53.5L233.7 0H512v189.3L466.3 257l45.7 65.8V512H233.7l-68-50.7-65.5 50.7H0V322.8l51.4-68.5-51.4-65z"/><path fill="#eee" d="M100.2 0v189.3H0v33.4l24.6 33L0 289.5v33.4h100.2V512h33.4l30.6-26.3 36.1 26.3h33.4V322.8H512v-33.4l-24.6-33.7 24.6-33v-33.4H233.7V0h-33.4l-33.8 25.3L133.6 0z"/><path fill="#0052b4" d="M133.6 0v222.7H0v66.7h133.6V512h66.7V289.4H512v-66.7H200.3V0z"/></g></svg>

flags/hm.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><mask id="a"><circle cx="256" cy="256" r="256" fill="#fff"/></mask><g mask="url(#a)"><path fill="#0052b4" d="M256 .1h256V512H0V256z"/><path fill="#eee" d="M154.4 300.4l14 29.4 31.8-7.3-14.2 29.3 25.5 20.2-31.8 7.2.1 32.5-25.4-20.3-25.4 20.3v-32.5L97.4 372l25.5-20.2-14.2-29.3 31.7 7.3zm228.9 55.7l7 14.7 15.9-3.7-7.1 14.6 12.7 10.2-15.9 3.5v16.3l-12.6-10.2-12.7 10.2v-16.3l-15.9-3.5 12.8-10.2-7.1-14.6 15.9 3.7zm-65.4-155.8l7 14.7 16-3.6-7.2 14.6 12.8 10-16 3.7.1 16.3-12.7-10.2-12.7 10.2v-16.3l-15.8-3.6 12.7-10-7-14.7 15.8 3.6zm65.4-89l7 14.7 15.9-3.7L399 137l12.7 10-15.9 3.7V167l-12.6-10.2-12.7 10.2v-16.3l-15.9-3.6 12.8-10.1-7.1-14.7 15.9 3.7zm57 66.8l7 14.7 16-3.7-7.1 14.7 12.7 10-15.9 3.7v16.2l-12.6-10.1-12.7 10v-16.1l-15.9-3.6 12.8-10.1-7.1-14.7 15.8 3.7zM399.6 256l5.6 17H423l-14.5 10.4 5.5 17-14.5-10.5-14.4 10.5 5.5-17-14.5-10.5h18zM256 .1H100.2L66.7 15 33.4 0H0v33.4l15.7 31.3L0 100.3v155.8h33.4l34-13.6 32.8 13.6h33.4v-75.2l75.2 75.2H256l-7.7-21.3 7.7-10.2v-15.7l-75.2-75.2H256v-33.4l-11.8-33.4L256 33.5z"/><path fill="#d80027" d="M256 224.6l-91-91h-31.4L256 256.1z"/><path fill="#d80027" d="M33.4.1v33.4H0v66.8h33.4v155.8h66.8V100.3H256V33.5H100.2V.1z"/></g></svg>

flags/sj.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><mask id="a"><circle cx="256" cy="256" r="256" fill="#fff"/></mask><g mask="url(#a)"><path fill="#d80027" d="M0 0h100.2l66.1 53.5L233.7 0H512v189.3L466.3 257l45.7 65.8V512H233.7l-68-50.7-65.5 50.7H0V322.8l51.4-68.5-51.4-65z"/><path fill="#eee" d="M100.2 0v189.3H0v33.4l24.6 33L0 289.5v33.4h100.2V512h33.4l30.6-26.3 36.1 26.3h33.4V322.8H512v-33.4l-24.6-33.7 24.6-33v-33.4H233.7V0h-33.4l-33.8 25.3L133.6 0z"/><path fill="#0052b4" d="M133.6 0v222.7H0v66.7h133.6V512h66.7V289.4H512v-66.7H200.3V0z"/></g></svg>

flags/um.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><mask id="a"><circle cx="256" cy="256" r="256" fill="#fff"/></mask><g mask="url(#a)"><path fill="#eee" d="M0 256L256 0h256v55.7l-20.7 34.5 20.7 32.2v66.8l-21.2 32.7L512 256v66.8l-24 31.7 24 35.1v66.7l-259.1 28.3L0 456.3v-66.7l27.1-33.3L0 322.8z"/><path fill="#d80027" d="M256 256h256v-66.8H236.9zm-19.1-133.6H512V55.7H236.9zM512 512v-55.7H0V512zM0 389.6h512v-66.8H0z"/><path fill="#0052b4" d="M0 0h256v256H0z"/><path fill="#eee" d="M15 14.5L6.9 40H-20L1.7 55.8l-8.3 25.5L15 65.5l21.6 15.8-8.2-25.4L50.2 40H23.4zm91.8 0L98.5 40H71.7l21.7 15.8-8.3 25.5 21.7-15.8 21.7 15.8-8.3-25.4L142 40h-26.8zm91.9 0l-8.3 25.6h-26.8l21.7 15.8-8.3 25.5 21.7-15.8 21.6 15.7-8.2-25.3 21.7-16H207zM15 89.2l-8.3 25.5H-20l21.7 15.8-8.3 25.5L15 140l21.6 15.7-8.2-25.3 21.7-16H23.4zm91.8 0l-8.3 25.5H71.8l21.7 15.8-8.3 25.5 21.7-15.8 21.6 15.7-8.2-25.3 21.7-16h-26.8zm91.8 0l-8.3 25.5h-26.8l21.7 15.8-8.3 25.5 21.7-15.8 21.6 15.7-8.2-25.3 21.7-16H207zM15 163.6l-8.3 25.5H-20L1.6 205l-8.3 25.5L15 214.6l21.7 15.8-8.3-25.4 21.7-15.9H23.3zm91.8 0l-8.3 25.5H71.7L93.4 205l-8.3 25.5 21.7-15.8 21.7 15.8-8.3-25.4 21.7-15.9h-26.8zm91.8 0l-8.3 25.5h-26.8l21.7 15.8-8.3 25.5 21.7-15.8 21.7 15.8L212 205l21.7-15.9H207z"/></g></svg>