This commit removes WILDCARD_MENTION_POLICY_STREAM_ADMINS
option of wildcard_mention_policy since we are not moving
forward with stream administrator concept and instead working
on new permssions model as per #19525.
We also add a migration to change wildcard_mention_policy of
existing realms to WILDCARD_MENTION_POLICY_ADMINS. This change
is fine since we were already treating both the setting values
as same as stream admin concept was not implemented completely.
Sahil Batra