Make extraction of attachments for SearchableText optional

[lukasgraf]

Currently, any attachments of a mail message get extracted by ftw.mails own SearchableTextExtender.

This is error prone, requires hacks for broken MTAs / MUAs and non-trivial to debug. In setups that already use ftw.tika it's also unnecessary - Tika already extracts full text from mail attachments, and probably does so in a much more reliable way than we can.

I would therefore propose to make this behavior in ftw.mail configurable via a registry entry (defaulting to True (extract attachments)).

An alternative approach would be to check whether ftw.tika is installed, and simply skip the extraction of attachments if it is.

@jone @maethu thoughts?

[jone]

The only use case I see for disabling the behaviour is tika.
Therefore I'd not make it configurable but explicitly check for an installed ftw.tika (unless you see other use cases)...

[lukasgraf]

@tarnap could you look into this if you have some time on your hands?

[phgross]

In GEVER we'll overwrite the SearchableTextExtender anyways (see 4teamwork/opengever.core#2462 (comment)), so there is no need to change the SearchableTextExtender for us.

@jone @maethu if you need this adjustments anyways please coordinate with @tarnap. For now he will not implemented this change.