RFR: 1903: Verify User's repository access when processing backport command [v3]

[Zhao Song]

> In GitLab, every project is under a group. If a user doesn't have access to the group, then the user will not be able to see any project under the group. 
> 
> However, when processing backport command, the bot will not verify user's group membership, so that it's possible for the bot to create a pull request that is invisible to the user. 
> 
> For example, if a user has access to "groupA" but not "groupB", then he can issue the "/backport groupB/repo2" command on one of the commits in "groupA/repo1". In this case, Skara bot would create a PR that is invisible to the user.
> 
> To fix this issue, we need to verify user's membership after we get the targetRepo. `GitLabRepository#canPush` is very helpful.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  fix some problems

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1516/files
  - new: https://git.openjdk.org/skara/pull/1516/files/0bb6a3a6..bb3546f8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1516&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1516&range=01-02

  Stats: 43 lines in 6 files changed: 29 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1516.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1516/head:pull/1516

PR: https://git.openjdk.org/skara/pull/1516