Fix skin-toned ZWJ emoji URL construction

ComposeSkinTonedCodepoints was appending the skin-tone modifier at
the end of the codepoint sequence. For ZWJ emojis like
person_running_facing_right (U+1F3C3 U+200D U+27A1 U+FE0F) this
produced URLs like 1f3c3-200d-27a1-fe0f-1f3fb.png which 404 on
Slack CDN. The CDN expects the modifier right after the base
codepoint, before the ZWJ: 1f3c3-1f3fb-200d-27a1-fe0f.png.

Insert the modifier at index 1 instead of appending at the end.