Add assign command

[burlakov-nick]

Sometimes it's handy to assign specific person to skip someone in the rotation (e.g. they're sick or on vacation). It can be done with several rotate commands, but it's not convenient.

Command could look like this

dienstplan assign <rota> user

[pilosus]

Sounds good. The command's convention is also in line with what's been done so far:

@dienstplan assign <rota-name> <user-mention>

The expected behav is the following. Let's say we've got a rota called duty with user list @user1 @user2 @user3 @user4. If the current oncall person is @user1 and we tell to the bot @dienstplan assign duty @user3, then @user3 becomes a current on-call and the order of users remains as it was.

Feel free to submit a PR.

[burlakov-nick]

Feel free to submit a PR.

I'll try, feel free to assign "assign" issue on me :)

[pilosus]

Done