Refactor reactions

Currently we effectively create the reactions list in EventHandler then pass that data into a model. This reworks the model so that we just pass in a room and an event and it grabs it's own data. This means that:
- the functions in event handler are no longer required
- the model can update itself to add/remove reactions so no need to handle that in MessageEventModel
- MessageEventModel only needs to create new ReactionModels or remove old ones when no reactions exist anymore

A basic test suite has also been created for the ReactionModel

autotests/CMakeLists.txt

@@ -70,3 +70,9 @@ ecm_add_test(
     LINK_LIBRARIES neochat Qt::Test
     TEST_NAME pollhandlertest
 )
+
+ecm_add_test(
+    reactionmodeltest.cpp
+    LINK_LIBRARIES neochat Qt::Test
+    TEST_NAME reactionmodeltest
+)