fix multiple bugs caused by launchMode="singleInstance" and noHistory="true"

following lines for ChatActivity were removed:
  android:launchMode="singleInstance"
  android:noHistory="true"

this solves:
- share files from local storage broken
- broken transitions (caused by "singleInstance")
- fix to open chat when navigating back from conversation info
- fix weird mixed conversations when opening a conversation by notification while being in another conversation

launchMode="singleInstance" was introduced when reimplementing breakout rooms for the conductor to activity migration. So for now this reverting might introduce problems for breakout rooms.

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

app/src/main/AndroidManifest.xml

@@ -211,8 +211,6 @@
 
         <activity
             android:name=".chat.ChatActivity"
-            android:launchMode="singleInstance"
-            android:noHistory="true"
             android:screenOrientation="portrait"
             android:theme="@style/AppTheme" />