[server] webui DB import and export

[feiloo]

addresses #11718

• uses the dexie addon "dexie-export-import"
• import wipes and refills the db, it doesnt add to the db
• import and export implemented as buttons
• reloads window after import to show the imported conversations

[ngxson]

I'm a bit concern that this is import/export at raw database level, not at DAO level.

This can be problematic if we introduce db migration in the future. For example, if the export is from an older version and it's imported in a newer version of the code, it will be messed up

[feiloo]

The solution and this PR depends on how much we want to rely on Dexie.

Dexie and dexie-export-import track the database version afaict, so it would error on incompatible db imports. It also enables database migrations: https://dexie.org/docs/Tutorial/Design#database-versioning

Otoh, Dexie probably also isn't bug-free in this regard, and more abstractions/dependent.
If Dexie-based import-export is too abstracted/debtful feel free to close without merge.

I tend to prefer fewer abstractions (no Dexie, just IndexedDB, lower-level import, export, migrations), but since Dexie was already used, i went with it for this PR.