refactor(appinfo)!: improve application models

[shiftinv]

Summary

Refactors application models, deduplicating and generally merging related code.

The diff is fairly unreadable as-is, skipping the first commit (that just moves PartialAppInfo above AppInfo) improves readability quite a bit.

tl;dr on the important changes, see changelog for everything:

• new hierarchy: PartialAppInfo < AppInfo < BotAppInfo (where BotAppInfo is equivalent to the old AppInfo)
  • note: this shouldn't have any practical effect on user code unless these types are created there
• AppInfo.summary is no more (deprecated in previous version)
• Invite.target_application (which is now AppInfo) contains more fields
• now using the correct cover_image url format

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running task lint
  • [x] I have type-checked the code by running task pyright
• [x] This PR fixes an issue
• [x] This PR adds something new (e.g. new method or parameters)
• [x] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[onerandomusername]

@shiftinv would you please resolve conflicts?

[shiftinv]

would you please resolve conflicts?

@onerandomusername done.

[onerandomusername]

@shiftinv would you please resolve conflicts again?