Fractal-server

* Allow setting `UserSettings` attributes to `None` in standard/strict PATCH endpoints (\1814).

* App (internal):
* Remove `FRACTAL_SLURM_SSH_HOST`, `FRACTAL_SLURM_SSH_USER`, `FRACTAL_SLURM_SSH_PRIVATE_KEY_PATH` and `FRACTAL_SLURM_SSH_WORKING_BASE_DIR` from `Settings` (\1804).
* Database:
* Drop `slurm_user`, `slurm_accounts` and `cache_dir` columns from `UserOAuth` (\1804)

> WARNING: This release requires running `fractalctl update-db-data` (after `fractalctl set-db`).

* API:
* Introduce user-settings API, in `/auth/users/{user_id}/settings/` and `/auth/current-user/settings/` (\1778, \1807).
* Add the creation of empty settings to `UserManager.on_after_register` hook (\1778).
* Remove deprecated user's attributes (`slurm_user`, `cache_dir`, `slurm_accounts`) from API, in favor of new `UserSetting` ones (\1778).
* Validate user settings in endpoints that rely on them (\1778).
* Propagate user settings to background tasks when needed (\1778).
* Database:
* Introduce new `user_settings` table, and link it to `user_oauth` (\1778).
* Internal:
* Remove redundant string validation in `FractalSSH.remove_folder` and `TaskCollectCustomV2` (\1810).
* Make `validate_cmd` more strict about non-string arguments (\1810).

* App:
* Replace `fractal_ssh` attribute with `fractal_ssh_list`, in `app.state` (\1790).
* Move creation of SSH connections from app startup to endpoints (\1790).
* Internal
* Introduce `FractalSSHList`, in view of support for multiple SSH/Slurm service users (\1790).
* Make `FractalSSH.close()` more aggressively close `Transport` attribute (\1790).
* Set `look_for_keys=False` for paramiko/fabric connection (\1790).
* Testing:
* Add fixture to always test that threads do not accumulate during tests (\1790).

* API:
* Make `WorkflowTaskDumpV2` attributes `task_id` and `task` optional (\1784).
* Add validation for user-provided strings that execute commands with subprocess or remote-shell (\1767).
* Runner and task collection:
* Validate commands before running them via `subprocess` or `fabric` (\1767).

> WARNING: This release has a minor API bug when displaying a V2 dataset with a history that contains legacy tasks. It's recommended to update to 2.5.1.

This release removes support for including V1 tasks in V2 workflows. This comes
with changes to the database (data and metadata), to the API, and to the V2
runner.

* Runner:
* Deprecate running v1 tasks within v2 workflows (\1721).
* Database:
* Remove `Task.is_v2_compatible` column (\1721).
* For table `WorkflowTaskV2`, drop `is_legacy_task` and `task_legacy_id` columns, remove `task_legacy` ORM attribute, make `task_id` required, make `task` required (\1721).
* API:
* Drop v1-v2-task-compatibility admin endpoint (\1721).
* Drop `/task-legacy/` endpoint (\1721).
* Remove legacy task code branches from `WorkflowTaskV2` CRUD endpoints (\1721).
* Add OAuth accounts info to `UserRead` at `.oauth_accounts` (\1765).
* Testing:
* Improve OAuth Github Action to test OAuth account flow (\1765).