Load custom certificates from an application-specific directory

[Olivier Tilloy]

According to Jamie in https://bugs.launchpad.net/oxide/+bug/1260048/comments/10:

  « Furthermore, upstream will be moving away from nss at some point anyway. For the time being we can initialize nss without user db. »

Currently oxide patches chromium to disable loading a shared user nss database from $HOME/.pki/nssdb.

Whether it is using NSS or another mechanism, we are going to need a way to manage custom certificates in oxide, for webbrowser-app to offer a UI to do that.

As a temporary solution until we add a proper API for certificates management, it would be useful to allow oxide to load custom certificates from an application-specific place (this would at least help fixing bug #1505995).