RFR: 2141: Inform user about accepting fork repo invite in /backport command reply [v2]

[Zhao Song]

> After users issued /backport command to create backport PRs in GitHub, skara bot will invite the users to be collaborators of the bot's fork. Sometimes, the users will ignore the invitation and later when they are trying to update the backport branch, they will find they don't have the access. This patch is trying to add a comment to remind the user to accept the invitation.

Zhao Song has updated the pull request incrementally with two additional commits since the last revision:

 - update
 - only send the reminder when user doesn't have the push access

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1598/files
  - new: https://git.openjdk.org/skara/pull/1598/files/1ac34cb5..d4df3bb3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1598&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1598&range=00-01

  Stats: 16 lines in 1 file changed: 6 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1598.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1598/head:pull/1598

PR: https://git.openjdk.org/skara/pull/1598