Allow for a custom congratulations template message for the First PR Merged Send Tweet GitHub action

[nickytonline]

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As a project maintainer, I want the ability to specify a message template for the First PR Merged Send Tweet GitHub action so that I can customize the congratulations Tweet.

Describe the solution you'd like A clear and concise description of what you want to happen.

For the First PR Merged Send Tweet GitHub action, a project maintainer should be able configure the GitHub action to point to a message template.

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This requires #4 to be implemented first.

Discussed in https://github.com/nickytonline/epic-actions/discussions/2

^{Originally posted by **nickytonline** October 7, 2021} @mtfoley, I thought one of one thing that might be nice. We can have a default template that sends out a pretty good congrats message by default, but folks may want to customize the message, hence a message template. Thoughts?

[mtfoley]

currently implemented in https://github.com/nickytonline/epic-actions/blob/55f26e39999953d686f9fc02924df35aa8207642/tweet-first-merge/action.yml#L16-L27

[nickytonline]

This looks great @mtfoley! As mentioned in DMs, let's use PRs instead of direct commits. I'll update the repo settings.