Lock issue mirrors to prevent comments by confused users

[cf-tm-bot]

As suggested by @voelzmo in http://cf-dev.70369.x6.nabble.com/cf-dev-FW-issue-tracker-permissions-tp2763p5812.html people sometimes get confused for mirrors of PT stories associated with GH issues or PR. They then post comments on the mirror issues instead of the original one.

This happened twice so far:

• https://github.com/cf-tm-bot/identity/issues/79#issuecomment-249992064
• https://github.com/cf-tm-bot/loggregator/issues/1#issuecomment-249329820

Original idea was to not prevent issue cross-references as preserve discovery, see #1 (comment)

Locking issues would help reduce this confusion. This could be phased:

• for all mirrors as a 1st step
• for mirrors of stories associated with GH issue/PR (as to preserve future support for Mirror PT user id to github id to enable @mentions to PT collaborators #8 ) as a 2nd step