Add Slackbot shortened commands

[evan-scott]

Is your feature request related to a problem? Please describe.
TCEU wants to type 'ar [resource]' rather than 'access request [resource]', partially because it's a lot to type and partially because they're envisioning this in a production incident scenario where every second counts.

Describe the solution you'd like
Allow accessbot to understand both 'access request' and 'ar' as suitable commands, along with any others that can be shortened.

Describe alternatives you've considered
N/A, open to suggestions though.

Additional context
https://app.gong.io/call?id=8746579382246779781&highlights=%5B%7B%22type%22%3A%22SHARE%22%2C%22from%22%3A1057%2C%22to%22%3A1791%7D%5D

IMPORTANT: Remember to select a label and project - I can't :(

[camposer]

Allow the user to configure command aliases via configuration:

COMMAND_ALIASES = {
  "show_resources": "sr",
  "access_resource": "ar",
  "assign_role": None,
  "show_roles": None,
  "approve": None
}

[camposer]

We ended up adding the aliases to the COMMANDS_ENABLED variable. I usually go with:

SDM_COMMANDS_ENABLED="access_resource:ar assign_role show_resources:sr show_roles approve:y deny"

So the alias for show available resources is sr