Add PluralKit import, PMP backup/restore/wipe functionality

[j0lol]

Description

Adds the ability to mass-manage personas. Also adds the ability to import personas from third party 'Pluralkit', to make usage of Sable in replacement/alongside Discord easier for the user.

BLOCKED BY #1621

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[j0lol]

• need to add avatar uploading
• record pk avatarurl in net.f0rest.pkimport so unnecessary reuploads don't happen

[j0lol]

whew. finally on main and free from being dependent on a pr