Fixes <Image> component URLs

[samglover]

This fixes bugs introduced or carried forward by #67 and #63 (on line 12 of the NavigationBar component, specifically).

Image file URLs that were working on my local environment were broken on the staging site, and vice-versa. I tried various relative paths to get around the problem, which didn't work because the staging site is served from a sub-directory of the domain, projects.suffolklitlab.org/courtformsonline.org, while the local environment was served from localhost:3000 with no subdirectory. So the relative paths were different on local v. staging environments and it was always broken on one or the other.

The Next.js basePath documentation led me to the solution. I added the basePath property to next.config.js so that the local and staging environments matched, imported the NextConfig object into files containing <Image> components, and inserted the basePath variable into the src property of the existing <Image> components.

When we change the domain to courtformsonline.org, we will need to update or remove the basePath property in next.config.js (either will work). The basePath import and the basePath segment in <Image> component src properties can be removed, as well (but will work fine if they are left in place).

I will update the launch checklist (#50) with those steps.

[samglover]

One more note. It looks like @jtmst was using a local environment variable to add /courtformsonline.org to the basePath only on his local environment. I think it's easier to detect broken file paths if the local environment matches the staging site more closely, so I prefer this approach.