[FEATURE] Ditch rota name in command syntax when there's only one rota

[burlakov-nick]

Is your feature request related to a problem? Please describe.
I use dienstplan in context when only one rota is created in slack channel. Usually I forget to specify rota name and type something like this:

@dienstplan assign @username

And get usage help message. I'm frustrated, because in my case dienstplan could understand that there's only one rota and allow ditching rota name in assign command syntax.

Describe the solution you'd like
I'd like to have a simplified syntax for assign command that works when there's only one rota in channel.

@dienstplan assign @username

Probably other commands can work the same way as described:

• rotate
• who
• shout
• about
• delete

Describe alternatives you've considered
As an alternative I could do my homework and memorize command syntax 🤓

[pilosus]

I've realized that we discussed it elsewhere, but I never replied here :-) So, for transparency:
I do think it's a nice feature!

Basically, in all the commands that use <rotation name> we need to do:

• check if rotation name is nil
• if it's nil, check in the db's rota table if there is exactly one rotation in the current slack channel, if yes, pass its name as rotation name.

This is probably not too tedious to implement, though it will affect a few moving parts:

• validation with spec
• parse-args multimethod

For me this feature isn't of any visible priority, as I find current syntax for dienstplan commands to be pretty straightforward: mention the bot -> command name -> rotation name -> all other arguments to the command (if any).

But I'll happy to review a PR.