Add slash command to delete files

[jecolvin]

Now that we're back on the Free plan, we need an easy way to delete old files to avoid hitting the
5 GB limit.

Using this gist as a starting point, the new slash command should delete files stored in Slack, while allowing the user to choose:

1. File type to delete
2. Age of files to delete (e.g., delete all files older than 6 months)
3. Limit to deleting files in a specific channel, if desired

Supporting documentation:

• files.list method
• files.delete method
• Handling tombstoned files
• Slash command docs