create reviewer assignment algorithm

[kaxada]

[kaxada]

@ElizabethN do you think this issue is still valid to solve? we are looking at the bot deciding who to assign a review to depending on whether they are available and have no issue assigned to them at the moment.

my argument is that there could be applications that need particular people to work on them so may need manual assignment because the algorithm won't know how critical a particular review issue is. do you think we can neglect that and go on with the automation?