Move static files out of pretix again, compile on setuptools build

src/static/bootstrap/less/mixins/text-overflow.less

@@ -0,0 +1,8 @@
+// Text overflow
+// Requires inline-block or block for proper styling
+
+.text-overflow() {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}