Remove references to Bugzilla

Replace any such references with references to GitHub, as appropriate.
Leave "dw-bugzilla@" email addresses alone.
Remove last bugs.dwscoalition.org references

Fixes #663. Both the references here removed were links to further docs
of what had been done, rather than notices about outstanding bugs, in
any case.
Actually fixes #663

Add a temporary explanatory note about the broken link to the CLA, and
what to do about it.
Change names of updates strings

Avoid later trouble in this way.  Simply append "2" in two places, in
each of two files, to fix commit before last.
(Issue #691) Hide 'track thread' icon from free users

Checks whether a user can track threads before displaying tracking icon.
(Issue #661) Allow People with Siteadmin Priv to PM Uncomfirmed Accounts

Allows somebody to PM an unvalidated account (unconfirmed email address)
if they have the siteadmin:* priv. Fixes #661.
Merge pull request #704 from hotlevel4/bug661/PMUnverifiedAccounts

(Issue #661) Allow People with Siteadmin Priv to PM Uncomfirmed Accounts
Merge pull request #703 from hotlevel4/bug691/hideTrackButton

(Issue #691) Hide 'track thread' icon from free users
Display poster column only when relevant

* in members/new, only show the poster column if the community post
  level is "select", instead of always showing (that was left over from
  when you could always revoke poster access, even if the post level was
  "all")

* in members/edit, always show the poster column if anyone has "posting
  access" set on them (this makes it act the same as
  unmoderated/moderator columns). This is a bit of an edge case, caused
  when someone switches community settings, but their community members
  used to have posting access explicitly -- this meant that the members
  would still show up on the "edit community members" page, because they
  still had an edge connected to the community, but there might not be a
  checkbox to uncheck
Merge pull request #705 from afuna/handle-poster-column

Display poster column only when relevant
Merge pull request #698 from Darael/bug663-bz-to-ghi

Fix #663 - Bugzilla to GitHub Issues migration