Adjust submit labels to use built-in i18n handling #2257
I noticed this while working on the diary entries, so now I'm rolling it out more widely. In many cases we've been avoiding using the built-in translation keys for submit buttons, and hard-coding translations based on the file the form is in (rather than based on the model for the form). This has made some form refactoring harder than it needs to be, as well as just being a bit of a waste of time. Sometimes this has been done just because of copying other places where it was done first.
There's ample opportunity to improve these labels in the future - too many saves and not enough updates in my opinion! But I'll leave the strings mostly unchanged for now.
@Nikerabbit this PR contains a few translation key renames: