Added checklist support #8085 merge req #227

[Shalitha Suranga <sh...@gmail.com>]

I have added checklist support [#8085]
<https://forge-allura.apache.org/p/allura/tickets/8085/> and these files
are changed

requirements.txt - added markdown checklist extension
app_globals.py - added markdown extension to ext array

How sub issues were solved

short reference links are affecting and instead checklist there were links.
Thus I modified
markdownextensions.py's ForgeLink Pattern

checkbox was not rendered to do sanitize function. Therefore I commented
input element from blacklist of util.py

Please review and suggest your ideas

merge request :
https://forge-allura.apache.org/p/allura/git/merge-requests/277/

-- 
Regards,

*Shalitha Suranga Bsc. CS (UG)*
www.shalithasuranga.me