Ability to search for cross references to sections of other RFCs and tag them with xref

[ajeanmahoney]

Sometimes an author will mention a section in an other RFC (e.g., "See Section 1.1 of RFC 9110") but not add a <xref> tag. The RPC will add <xref> tags when we find these mentions.

The user should be able tag these section mentions with <xref> automatically by selecting an option from a menu or by clicking a button. Like other Find/Replace features, this should allow the user to either confirm each update or to permit changes throughout the XML file.

This functionality is currently provided by the fixsubref.py script.