Previously, the value for an option of the select type custom
profile field was set as "order - 1". This commit changes it
to remain same even when we reorder the options or delete an
option. When we add a new option, its value is set as 1 more
than largest value already used.
This helps is eliminating various bugs in this subsystem, where
user's choice is changed unexpectedly when reordering or deleting
options.
Discussion -
https://chat.zulip.org/#narrow/stream/378-api-design/topic/custom.20profile.20fields.20option.20deletion.