Add P### link to paste
Summary:Make a new directory, src/infrastructure/markup/remarkup/markuprule/paste/
Make a new class called PhabricatorRemarkupRulePaste in that directory.
Add the rule to DifferentialMarkupEngineFactory.
Test Plan: Created a task in maniphest. Put P1 and P2 in the content.
Created P1 and P2 in Paste. Verified P1 and P2 were highlighted and
linked correctly.
Reviewers:epriestley, codeblock
CC:jungejason
Differential Revision: 539