Minor phutil_tag/HTML fixes

Summary:

  - In stack traces, a `,` should clearly be a `.`.
  - In Calendar, a 'td' got swapped with a 'p' somewhere.
  - In old-style transaction views, strlen() is no longer a sufficient test.

Test Plan:

  - Verified stack traces render correctly.
  - Verified calendar renders correctly.
  - Verified Maniphest transactions with no comment no longer have a little empty div a few pixels high.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4971