Seafile: allow non-local MySQL installations

[melvyn2]

Describe the bug

The current services.seafile module enables and connects to a MySQL (mariaDB) server on the host.
However, it should also support connecting to a remote MySQL instance instead,

Steps To Reproduce

Steps to reproduce the behavior:

1. Enable the seafile module
2. Note that the mariaDB server started and cannot be disabled without breaking seafile

Expected behavior

The module provides configuration options to select and authenticate to a remote database,
and doesn't enable the local mariaDB instance if these are enabled.

Additional context

Seafile used to use a local SQLite database which had no options to connect to a remote database.
As part of the migration to MySQL in #318727, no options for remote connections were added,
due to the complexity of handling these options for the 3 services which require them
(in different formats), as well as the difficulty of properly passing secrets (MySQL passwords)
to each service.
A work-in-progress patch to add this functionality was posted in #318727 (comment)

Notify maintainers

@greizgh @schmittlauch

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