Add formal leadership roles to committees

@DSASanFrancisco/portal-members

Summary

Specs:

• https://docs.google.com/document/d/1za2laXnpPMLmCEOIs6sOUSqYPcn-h8-SIRX4KfZ_SqQ/edit
• https://docs.google.com/document/d/1PJ58FknB3hlGpx17ETMNh10u030t6ll7W2ClBFvaDt8/edit

Specs summary:

• New formalized leadership roles on committees, with titles and term limits
• Add new custom role titles
• Add multiple copies of the same type of leadership role to the committee
• Set which members hold which leadership roles