Use Helix for loading Emote Set info

[pajlada]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

• Create the *helix.Client in an internal package, making it passable to multiple packages that may want to use the same client.
• Start replacing twitchemotes.com calls with Helix Get Emote Sets calls

Changes:

• The /twitchemotes/sets/ endpoint was removed completely - it was never implemented nor used
• The nicklaw/helix API Client is now initialized in cmd/api/main.go and passed through to any users. This allows multiple packages to share the same client.
• We now create a "helix username" cache that can be used to resolve a Twitch ID to Twitch Username. Resolves are cached for 1h and are currently only used in the twitchemotes package
• Removed any custom emote set modifications we previously did
• For /twitchemotes/set/{setID}/ we only populate the bare minimum fields: channel_name and channel_id. The rest of the fields are still part of the JSON response, just left to their default values (empty string/0/false)
• Twitchemotes route code moved to its own internal package under /internal/routes/
• A trailing slash is no longer required for the /twitchemotes/set/{setID}/ endpoint

Closes #170
Closes #174

Talked this through already

[zneix]

🚀