feat[UnifiedPush]: notifications for multiple accounts

[baltevl]

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

• [x] Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• [x] The commit message uses the format of Conventional Commits
• [ ] The commit message describes what has been changed, why it has been changed and how it has been changed
• [ ] Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• [ ] Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• [x] Android (emulator only for now)
• [ ] iOS
• [ ] Browser (Chromium based)
• [ ] Browser (Firefox based)
• [ ] Browser (WebKit based)
• [ ] Desktop Linux
• [ ] Desktop Windows
• [ ] Desktop macOS

Hey, again this is another attempt to bring support for notifications for multiple accounts to fluffy. As with the previous one this is only a draft for now and I'd appreciate any feedback :)