[Xon] Image Count Limit by User Group 2.5.0

Require StandardLib v1.21.0+
Confirm XF2.3 support

Fix bimg tag from Advanced BbCode Pack would be counted twice for the minimum image count
Add "Exclude quoted images from image count" option (default disabled)

• Fix error when guest posts

• Require php 7.2+
• Require Standard Lib v1.18.0+
• Add "Minimum images for reply posts" and "Minimum images for first post" permissions
• Add "Apply minimum image count for threads with prefixes" option
• Add "Include attachments to the minimum image count" option

• Add empty placeholder installer file for upgrades from very old versions