Reviewed by Tony Chang.

        Rebaseline server: list current baselines and platforms
        https://bugs.webkit.org/show_bug.cgi?id=49991

        List existing baselines (with Trac links) for tests.

        Add dropdowns for choosing with platform baselines to target (and what
        to do with existing ones).

        Also fix MockFileSystem.join to behave more like os.path.join (unit
        test was ending up with two consecutive slashes in a layout test
        path).

        * Scripts/webkitpy/common/system/filesystem_mock.py:
        * Scripts/webkitpy/tool/commands/data/rebaselineserver/index.html:
        * Scripts/webkitpy/tool/commands/data/rebaselineserver/main.css:
        * Scripts/webkitpy/tool/commands/data/rebaselineserver/main.js:
        * Scripts/webkitpy/tool/commands/data/rebaselineserver/queue.js:
        * Scripts/webkitpy/tool/commands/data/rebaselineserver/util.js:
        * Scripts/webkitpy/tool/commands/rebaselineserver.py:
        * Scripts/webkitpy/tool/commands/rebaselineserver_unittest.py: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@72640 268f45cc-cd09-0410-ab3c-d52691b4dbfc