Sentry DSN - centralized cross-project definition

> Can we make this [the DSN constant] a global constant somewhere -- especially good if we can make it cross-language (this may not be achievable so at least all in the same location)... as it is shared between projects.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/2771_

> If possible, it should be in a separate file because it's just cleaner for future maintenance (global search and replace is of course not difficult but it's easy to forget a platform if the dsn is hidden away).

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/2771_

Note that the final numeral on the DSN, after the `/`, is project specific.

Defining this as a 'resource' similar to VERSION.md and TIER.md sounds like the goal, though there is then the issue of importing the value from the resulting file into the actual code of each of our platforms.

Another note:

> The ideal: have a /resources/sentry.properties JSON file which is consumed by all projects at compile time. This way we can specify details such as sentry server, project, DSN, etc in one place.
>
> The reality: this may not work out too well. No point in adding extra code because that is more to maintain.
>
> So it's a question only: if we can get an easy win DRYing out our references to the DSN, let's do it. Otherwise, we won't worry. I've asked @jahorton the same question.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/2778_