Try to install PostgreSQL extensions automatically (backport #1483)

[mergify]

This is an automatic backport of pull request #1483 done by Mergify.
Cherry-pick of 7ff40fb has failed:

On branch mergify/bp/master/pr-1483
Your branch is up to date with 'origin/master'.

You are currently cherry-picking commit 7ff40fb85.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/manage_pg.c

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 14614dc has failed:

On branch mergify/bp/master/pr-1483
Your branch is ahead of 'origin/master' by 3 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 14614dcde.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   INSTALL.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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