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