WIP: Make mail server logs searchable

[laurentS]

Relevant issue(s)

#8814

What does this do?

This makes MailServerLogs searchable via the new postgres based search system.

Why was this needed?

Implementation notes

See #9334 (comment) about speeding up this specific model's indexing, for the reasoning behind overriding the base reindex_all method.

Trimming log lines before indexing them will probably help with index size. The email log lines contain a lot of "trivial" text that is very unlikely to be searched (timestamps, server name which appears on every line, "postfix", etc...).
This would probably require a separate SQL function for each MTA.

Screenshots

Notes to reviewer

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[laurentS]

We might need to get rid of the overridden reindex_all method here if we merge the more generic one in #9439

[garethrees]

We might need to get rid of the overridden reindex_all method here if we merge the more generic one in #9439

Not sure if this is necessarily the best place for this comment, but just at a quick glance I wonder if we could have a setup where we have the generic reindex_all, but can then call a per-model custom implementation if defined:

class SomeRecord
  include Searchable
  # Uses the default reindex_all method
end

class AnotherRecord
  include Searchable

  def self.reindex_all
    # custom implementation
  end