Reviewed-by: Suwon Chae

To migrate, following options are required at application.conf

application.conf
----
github.allow.migration = false
github.client.id = "TYPE YOUR GITHUB CILENT ID"
github.client.secret = "TYPE YOUR GITHUB CILENT SECRET"

Now, project admin user(s) can export who's specific projects.

e.g.
Assume that user 'tester' is admin of project name 'sample'.
If tester call below url with Accept header with 'application/json'
and Yona-Token header with valid token, project issues and posts will be
download in format of JSON.

Accept: application/json
Yona-Token: dsflaksjflksadjflasjfaslfjds=

http://repo.yona.io/tester/sample/exports

Suport yona to github migration.
It can be processed at /migration url path.

eg. https://repo.yona.io/migration

Facebook OG tag support just minimal cases.

- Project Home, Issue/Board View Page

Reviewed-by: Suwon Chae

Add new contributor

'Issue' and 'Board' page won't be rendered properly if user input markdown header contains 'upload' text. 'upload' text is used by file upload form's tag id by hard coded keyword.

This commit adds markdown heading prefix not to conflict markup ids.

Reviewed-by: Suwon Chae