Change exit.label from a translated resource to an optional string, or remove it

[markboots]

Currently, block.exit.label is a translated resource, and is required for exits. This is inconsistent with flow.label and block.label, which are optional human-readable strings that expand on flow.name, block.name.

By making exit.label into an optional string, it would:

• Significantly reduce the number of resources required on a flow.
• Avoid the problem where we do not typically have a good input source of translated exit names from flow builder UIs.
• Create consistency with block.label and flow.label

Since exits are a flow logic element that are not visually exposed to contacts (end-users), we're not aware of places where translation for the exit name needs to be presented to contacts.

Proposal:

• Change block.exit.label to an optional string. exit.name to be used on flow displays if exit.label is not provided. This creates consistency in the usage of name and label on flows, blocks, and exits.

[markboots]

Going one step further, in discussions today we recognized that:

• There is no need to restrict exit.name to word-chars only (a variable name), since exit names are not used as keys in expressions. Therefore, exit names could have spaces, etc.
• With this in mind, there is no need or use-case we can currently see for an exit.label beyond exit.name. We'll update the PR to go one step further and remove exit.label.