interface_type select menu will be used to choose the interface
for outgoing webhooks. It will be displayed only when the selected
bot type is OUTGOING WEBHOOK type. The default value is GENERIC
interface type (1).
Add 'Type of bot' option for bots by adding dropdown option in
settings->"Your bots". For now, this allows creating incoming webhook
bots in addition to default bots.
This will enable users to add a bot as an incoming webhook
(in addition to add full-featured bots).
With various minor tweaks and cleanups by tabbott.
Fixes#2186.