Update charter given feedback in #2, #3, #6, #8, and #10. (#5)

* Update mission on home page to match the updated charter language.
* Address a whole bunch of the feedback from issues #2 and #3.
* Improve readability a bit.
* Tweak wording re: WebAppSec per Tanvi's review of #5.
* Chairs need to be able to close Proposals for any number of reasons (e.g. moderation).
* Chairs must announce the removal of Work Items with rational for removal.
* Explicitly reference the Ethical Web Principles from our mission statement.
* Move the list of Work Items closer to the beginning of the Work Items section.
* When migrating work to a WG, the Editors and Chairs will work together to figure out what destination is best.
* Spell out more of the Chairs' responsibilities.
* Linkify 'meetings' in a couple places.
* Drop sentence per #10.
* Spell out how Chairs formally notify the group of things.
* Try to prevent data loss when closing Proposals or removing Work Items.
* Use the non-draft link for the Ethical Web Principles.
* Drop the chair maximum.
* Contributions to Proposals are also under the CLA.
* fix typo