Support updating theme repos

[jristau1984]

When using an alternative theme such as https://github.com/edx/edx-themes, devstack's make dev.reset-repos doesn't update the checkout of the theme repo. This can lead to bugs during development as the themes and the code they call get out of sync. We should find a way to update the repos of themes that are in use.

(edx/edx-themes in particular is a bit of a special case since there are already references to it in devstack and edx-platform.)

Ideas:

• Just update the troubleshooting doc.
• Add a private list of additional repos for the devstack commands to update.

[huniafatima-arbi]

When we run the make dev.reset-repos we want the themes repos to be updated as well.
Theme repos are placed in src directory in project folder. As user can use any theme while following the theme guidelines. We will be maintaining a local file with theme repos list for each user. This file won't be pushed to remote. When the user runs the above command, it will look for that local file, read the repos list and look for those repos in src folder. If the repo exists we shall update it.
@jristau1984 please confirm if I understood the problem correctly and the solution aligns with your expectations.

[jristau1984]

Pinging @dianakhuang to confirm this approach.

[dianakhuang]

@huniafatima-arbi I think this is okay to only concern the edx-themes repo, regardless of other themes repositories that might be installed. I think we just want to check for the existence of the themes directory (hopefully added by default once edx/devstack#67 lands) and make sure it gets updated with make dev.reset-repos.