Additional bloc architecture improvements

[hjiangsu]

Pull Request Description

This PR is a continuation of previous work to help improve the overall bloc architecture. The main changes include:

• Improvements in session handling within the app. ProfileBloc has been split up into SessionBloc and ProfileBloc. SessionBloc handles initializing, adding, removing, and switching between accounts while ProfileBloc is used to handle the account's data (e.g., user information, subscriptions, favourites, etc.).
• Split up UserSettingsBloc into AccountSettingsCubit, UserBlocksCubit and UserMediaCubit to improve separation of concerns, and re-worked the settings pages to use the new blocs.
• Converted some blocs into cubits to simplify logic, and other miscellaneous changes related to session handling/switching and account resolution.

I've also included an experimental refactor of the image viewer which doesn't rely on extended_image (the eventual goal is to remove extended_image as a dependency). This is enabled when experimental settings are enabled. Additionally, I've removed the tests temporarily as I want to completely re-do the test suite to better catch regressions in the future.

There will likely be follow up PRs to further clean up the codebase but this has gotten pretty large already 😅

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?