[Feature Request] Allow DJ, Admin or Owner to add songs longer than "maxtime"

**Is your feature request related to a problem? Please describe.**
Currently, DJs, Admins or the Bot Owner cannot add a song if it is longer than `maxtime` in the config file.

**Describe the solution you'd like**
I would like a config option (e.g. `lengthoverride`) that could be set to DJ, Admin or Owner which would allow the respective roles (and roles above it) to add songs longer than `maxtime`.

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Set `lengthoverride=DJ`
2. Now as a DJ, Admin, or Owner, you can add songs longer than `maxtime`

**Have you confirmed that this idea is not already described here?**
https://github.com/jagrosh/MusicBot/projects/1
Yes

Start by locating the configuration handling for `maxtime` and the code that rejects songs exceeding it. Trace how DJ, Admin, and Owner roles are represented and checked. Done means a `lengthoverride` setting accepts the requested role levels and permits those roles and higher to add songs beyond `maxtime`, while lower roles remain restricted.

Written by the indexing model from the issue text.