internal_send_message: Enforce being passed cross-realm bots.
internal_send_message: Enforce being passed cross-realm bots.

We include ERROR_BOT in this set, even though it's not technically
cross-realm (it just lives in the admin realm).

This code path does not correctly handle emails that correspond to
multiple accounts (because `get_system_bot` does not).  Since it's
intended to only be used by system bots, we add an appropriate
assertion to ensure it is only used for system bots.

zulip/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,rishig/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,showell/zulip,brainwane/zulip,rht/zulip,mahim97/zulip,synicalsyntax/zulip,jackrzhang/zulip,andersk/zulip,shubhamdhama/zulip,rht/zulip,tommyip/zulip,jackrzhang/zulip,rishig/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,timabbott/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,dhcrzf/zulip,timabbott/zulip,brainwane/zulip,dhcrzf/zulip,eeshangarg/zulip,mahim97/zulip,shubhamdhama/zulip,rht/zulip,zulip/zulip,jackrzhang/zulip,jackrzhang/zulip,shubhamdhama/zulip,rishig/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,andersk/zulip,timabbott/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,jackrzhang/zulip,punchagan/zulip,tommyip/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,tommyip/zulip,hackerkid/zulip,brainwane/zulip,dhcrzf/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,mahim97/zulip,jackrzhang/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,tommyip/zulip,mahim97/zulip,jackrzhang/zulip,dhcrzf/zulip,rht/zulip,hackerkid/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,andersk/zulip,tommyip/zulip,tommyip/zulip,rishig/zulip,dhcrzf/zulip,hackerkid/zulip,showell/zulip,tommyip/zulip,rishig/zulip,andersk/zulip,zulip/zulip,mahim97/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,dhcrzf/zulip,synicalsyntax/zulip,zulip/zulip,dhcrzf/zulip,zulip/zulip,rishig/zulip,synicalsyntax/zulip,synicalsyntax/zulip,brainwane/zulip,rishig/zulip,kou/zulip,mahim97/zulip