chore: Implement short translation keys

[danjohnson95]

This PR adds translation keys for every string in the application, and does so in a consistent short-keys format.

All strings live inside the cachet namespace, and so are accessed like __('cachet::incident.new_button').

I've separated out the translation keys roughly by the feature (or "resource") that they relate to. For example any strings related to "Incidents" belong in the incidents.php file.

I've created a predictable naming structure for some of the common page designs, so the table headers used on resource list pages are in cachet::incident.list.headers.

I've tried to keep this change as small as possible, so this PR doesn't contain the functionality to switch to different locales, but I'm happy to do that as a separate change.

If you're happy with this change, I've got some further tooling I can implement to keep the codebase clean - ie tests to ensure we're only using keys that exist, and linting rules to enforce consistencies in translation files.

[jbrooksuk]

Woah, huge thanks @danjohnson95 for undertaking this mammoth task! 🙏

I'm going to merge this in now and I'd love for future work to provide the tooling you mentioned.