Rename [assign.custom-welcome-messages] and it's fields

[Urgau]

This PR renames (while keeping compatibility for the names) [assign.custom-welcome-messages] and it's fields to:

[assign.custom_messages]
auto-assign-someone = "Welcome message, assigning {assignee}!"
auto-assign-no-one = "Welcome message for when no reviewer could be found!"

Old syntax

[assign.custom_welcome_messages]
welcome-message = "Welcome message, assigning {assignee}!"
welcome-message-no-reviewer = "Welcome message for when no reviewer could be found!"

This is done in order to be less confusing to users as "welcome" can be confused with a first time welcome message instead of the meaning we use, namely the welcome message of the PR. I also took the opportunity to clarify the fields meaning (in particular regarding manual-assign).

Reported at rust-lang/miri#4472 (comment).

Only the last commit is relevant.