[Utils] Finish and Refactor Predicate Utility (#2169)

* Uses classmethods to create predicates
* Classmethods allow using a combination of different parameters to describe context
* Some predicates assign a captured `result` to the predicate object on success
* Added `ReactionPredicate` equivalent to `MessagePredicate`
* Added `utils.menus.start_adding_reactions`, a non-blocking method for adding reactions asynchronously
* Added documentation
* Uses these new utils throughout the core bot
Happened to also find some bugs in places, and places where we were waiting for events without catching `asyncio.TimeoutError`

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

redbot/cogs/downloader/checks.py

@@ -1,7 +1,7 @@
 import asyncio
 
-import discord
 from redbot.core import commands
+from redbot.core.utils.predicates import MessagePredicate
 
 __all__ = ["do_install_agreement"]
 
@@ -21,13 +21,12 @@ async def do_install_agreement(ctx: commands.Context):
     if downloader is None or downloader.already_agreed:
         return True
 
-    def does_agree(msg: discord.Message):
-        return ctx.author == msg.author and ctx.channel == msg.channel and msg.content == "I agree"
-
     await ctx.send(REPO_INSTALL_MSG)
 
     try:
-        await ctx.bot.wait_for("message", check=does_agree, timeout=30)
+        await ctx.bot.wait_for(
+            "message", check=MessagePredicate.lower_equal_to("i agree", ctx), timeout=30
+        )
     except asyncio.TimeoutError:
         await ctx.send("Your response has timed out, please try again.")
         return False