Support updating theme repos

[timmc-edx]

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.)

[robrap]

Ideas:

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