[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/warnings/warnings.py

@@ -15,6 +15,7 @@ from redbot.core.i18n import Translator, cog_i18n
 from redbot.core.utils.mod import is_admin_or_superior
 from redbot.core.utils.chat_formatting import warning, pagify
 from redbot.core.utils.menus import menu, DEFAULT_CONTROLS
+from redbot.core.utils.predicates import MessagePredicate
 
 _ = Translator("Warnings", __file__)
 
@@ -363,12 +364,11 @@ class Warnings(commands.Cog):
         """Handles getting description and points for custom reasons"""
         to_add = {"points": 0, "description": ""}
 
-        def same_author_check(m):
-            return m.author == ctx.author
-
         await ctx.send(_("How many points should be given for this reason?"))
         try:
-            msg = await ctx.bot.wait_for("message", check=same_author_check, timeout=30)
+            msg = await ctx.bot.wait_for(
+                "message", check=MessagePredicate.same_context(ctx), timeout=30
+            )
         except asyncio.TimeoutError:
             await ctx.send(_("Ok then."))
             return
@@ -385,7 +385,9 @@ class Warnings(commands.Cog):
 
         await ctx.send(_("Enter a description for this reason."))
         try:
-            msg = await ctx.bot.wait_for("message", check=same_author_check, timeout=30)
+            msg = await ctx.bot.wait_for(
+                "message", check=MessagePredicate.same_context(ctx), timeout=30
+            )
         except asyncio.TimeoutError:
             await ctx.send(_("Ok then."))
             return