Emit a table of warnings by project and severity.

* Use 't1' CSS class for main warning tables,
  and default table style for new warning count table.
* Count total skipped warnings.
* Append count of warnings in each severity header.
* Fix some skipped warning patterns and count/dump skipped warnings.
* Add dumpskippedanchors function to fix expand_collapse
  error when --byproject flag is used and severity.SKIP
  is not emitted.
* Minor coding style changes:
  * Loop through severity.kinds.
  * Remove unused import.
  * Add spaces around assignment operators.
  * Adjust indentation.

Bug: 31377083
Test: run warn.py with build.log
Change-Id: I68949edc4601b10ea2f7ac84e48e6c3da4ab1e4f