[Feature Request] Skip commands to add to the back of the queue when repeat mode is on

**Is your feature request related to a problem? Please describe.**
We would like for the skip commands to push the currently playing song to the back of the queue when repeat mode is on so that the queue remains intact. Currently, it removes the song from the queue regardless of repeat mode status.

**Describe the solution you'd like**
Skip/Forceskip/Skipto should move the song to the back of the queue when repeat mode is on to keep the queue intact. Users can use remove to remove the song from the queue if necessary.

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Turn on repeat mode (!repeat on)
2. Type !skip
3. Currently playing song is added to the back of the queue, and the next song is played.
4. Turn off repeat mode (!repeat off)
5. Type !skip
6. Next song plays, previous song is not added into the queue.

**Additional context**
No other context

**Please check the following links to confirm that your feature has not already been requested:**
https://github.com/jagrosh/MusicBot/labels/enhancement
https://github.com/jagrosh/MusicBot/wiki/Things-That-Won%27t-Be-Added

Start by locating the handlers for !skip, !forceskip, and !skipto, then trace how repeat mode and the current queue are managed. Done means each skip command requeues the current song only while repeat mode is on, while normal skipping leaves it removed, matching the listed flow.

Written by the indexing model from the issue text.