data_import: Make slack bot emails unique.
data_import: Make slack bot emails unique.

Slack bot emails generated by us can be duplicate for two bots.
If such a case occur, append a counter to the email to make it
unique.

For maintaining the counter of duplicate emails and the final
email assigned to each bot, a class based approach is used with
static variables and static (class) methods. This keeps all the
data related to slack bot emails at the same place and easily
accessible from anywhere inside the module (without defining any
class object and passing it around).

Fixes: #16793

zulip/zulip,rht/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip