rose.suite_log_view: consolidate logic for GUIs.

Use the same logic to launch to web browser.
Document: fix typo in --cycle-offset=OFFSET syntax.
Merge pull request #480 from matthewrmshin/397.gui.suite_log_view

rose.suite_log_view: consolidate logic for GUIs. Closes #397
Merge branch 'master' into cylc.db