ACP option for hiding attachments

[marcovo]

Make an ACP option for hiding ACP attachments.

https://www.phpbb.com/community/viewtopic.php?p=14140856#p14140856

[marcovo]

Attempted fix in commit: 52ed482

This also includes a bugfix for a problem that was actually mentioned in the very same post, that attachments were hidden regardless whether a [hide] bbcode was present or not.

I did not yet test if it works with inline attachments.

[marcovo]

Since the latest commit 101bf13 , inline attachments will be hidden if they are inside [hide] tags. Other attachments (e.g. non-inline ones) are hidden whenever there is a [hide] bbcode present in the text.

This probably is the best solution for attachments. Other solutions might be more prone to errors or false positives.