Convert image 4.7

Convert image v4.7 changes:

Fixed issue with checking for WEBP support when using GD.

Convert image v4.6 changes:

Fixed issue with converting images in conversations.

Convert image v4.5 changes:

Attachment tables xf_attachment and xf_attachment_data are now updated more accurately to reflect original post date and post user id.

Convert image v4.4 changes:

Added 'Convert conversations' to options page.

Convert image v4.3 changes:

Added 'Exclude domains' to options page.

Convert image v4.2 changes:

Now retains IMG bbcode options.

Convert image v4.1 changes:

Now supports webp images by converting them to jpg.

Convert image v4.0 changes:

Added Extension check to options page.

Convert image v3.9 changes:

Code now only runs if post message is new or edited.

Convert image v3.8 changes:

Improved corrupt image error handling.