Only add extra_template_basedirs if it has not been set

[tmetzl]

In nbgrader v0.6.x we only set the template_path option if it is not already in the config (see here).
In nbgrader v0.7.x we should also only set the the extra_template_basedirs conditionally in order to not break any custom exporters.

[jhamrick]

Thanks @tmetzl !

Can you modify this to merge into master rather than 0.7.x? I think you just would need to cherry-pick your commit (so it excludes the 'Bump version to 0.7.1.dev' commit).

[tmetzl]

@jhamrick I think I messed by dropping the commit. Should I create a new clean PR in favor of this one?

[jhamrick]

I was able to fix it! 😄

(If you're curious, here are the git commands I used to do it)

# get the latest changes
git fetch upstream
# create a temp branch off of master to fix the PR
git checkout master
git branch patch-1-fix
# checkout the PR, then cherry-pick the relevent commit; this gives us a branch that only has the one commit in it, and not the others
gh pr checkout 1579
git checkout patch-1-fix
git cherry-pick 61df6a5
# reset the PR branch to be the same as the temp branch (throws away the other commits)
git checkout patch-1
git reset --hard patch-1-fix 
git push -f

[tmetzl]

Perfect 👍
Thank you for the git commands and the comments.

[jhamrick]

Tests are passing locally, so should be good to merge.

[jhamrick]

@meeseeksdev backport to 0.7.x