models: Add assert statement for full_members policy in has_permission.
models: Add assert statement for full_members policy in has_permission.

This commit adds an assert statement in the last block of
has_permission which checks whether the policy_value is
POLICY_FULL_MEMBERS_ONLY. This assert statement is added
for readability.

rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,rht/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,kou/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,kou/zulip