feat: Added __repr__ to all connectors + added tests

[TheCleric]

Addresses #461

Also did a little bit of cleanup here. If it's deemed unnecessary let me know and I'll remove it.

• Due to combining the get_server_url code and the other mastodon instance splitting code from self_url, it is now called get_handle_parts and it was moved to the connectors/mastodon.py file and tests were updated as well.
• Due to that change I wanted to add tests for self_url() to make sure everything worked as expected.
• While doing that it felt proper to have tests for post_url as well, which now leverages self_url to get its base URL. (Though I wonder if these should be implemented by their connectors instead so we don't have a lot of "business logic" in the channel as we continue to add more channels.

Sorry for the extra rabbit hole of stuff, I got a little carried away. 😆

[mlissner]

@TheCleric, @ERosendo is assigned to review this, but FYI, he has a couple things in line before this that are higher priority. I expect those may take a few days, but I'd guess you'll get a review this week.

Thank you!

[TheCleric]

Also while I was doing this it's obvious the BlueSky tests are behind Twitter and Mastodon, so I have some extra tests I added that I'll submit in a separate PR after this one is merged.

[mlissner]

I'll duck out. All yours to merge when you're ready, @ERosendo.