Add-ons Multi Prefix 2.16.3

• Update installer to better handle very large number of threads upgrading

• Remove leftover select2 css
• Recommend Standard Library by Xon v1.21.9+ due to prefixes may disappear when using the "move thread" dialog. Thanks @nvtpro

• Fix API support for @XenConcept's Project Manager not working as expected
• Support displaying multiple prefix descriptions in a thread