Avoid fetching or relying on guild member info

[Intery]

Issues

Due to recent discord API changes, the member cache is usually non-existent, and guild.get_member() will usually return None.

This is used, in particular, while loading timer subscribers from session data, resulting in no subscribers being loaded.

Proposed solution

Avoid using member objects wherever possible, relying solely on the user id.

[Intery]

Temporarily patched this by changing the get_member call into fetch_member, which queries the API for member information.
This significantly increases startup time and doesn't scale well, but allows a way around the issue for the moment.

[Intery]

Implemented in 9634af1