Rename "Downloading and processing profile" to "Loading and processing profile"

When importing a profile from a local server, we display "Downloading and processing profile..."
To uninitiated users, this can make it sound like the profile is somewhere on a public server, and we're downloading it from the network. But instead we're just loading it from the same machine.

I think we could just rename this to "Loading" instead of "Downloading" and it would fit in all cases where it's used at the moment. This would sound less alarming.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-499)