[#6464] Changed tool_label on GC code importer

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6464] Refactored plain2markdown to Allura helpers

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6464] Use plain2markdown to escape ticket description and comments in GC Issues importer

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6464] Added skip_mod_date to GC tracker importer

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6464] Fixed attribute error on missing / empty GC issue fields and added tests

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6464] Added tests for GC tracker attachments and comments

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>