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

[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.

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

Commit messages:
 - SKARA-2141

Changes: https://git.openjdk.org/skara/pull/1598/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1598&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2141
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 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