[DCom] Auto Merge Double Post 2.4.0

• Fixed a bug where messages would not merge when the message length limit was disabled.
• Fixed a bug in the regular expression where merging of quoted messages was prohibited.
• Added a check to ensure that the "[automerge]" tag contains nothing but a timestamp.
• Added handling of the maximum number of attachments in a message.
• Code refactoring.

• Fixed a bug when merging messages when a message is not sent on the last page.

Thanks @rdn for finding the error.

• Only Xenforo 2.3 support