Auto-unreact feature

When someone is picked in a giveaway their reaction is automatically removed from said giveaway (helps with rerolls). This feature can be used whenever u make a giveaway by adding "au" (or something else) after !gstart or as an option whenever you make a giveaway step-by-step

The issue names the !gstart command and the step-by-step giveaway flow; start by tracing those entry points and the winner-selection reaction handling. Done means an opt-in such as “au” removes a selected participant’s reaction in both creation modes, including rerolls. No files or tests are named, so locating those paths will require familiarity with the bot.

Written by the indexing model from the issue text.