init: add shared workflows and tools

[jnsgruk]

This PR adds a set of actions and tools that can be shared across Snapcrafters repositories. This should cut down the amount of boilerplate in each repo, and give us one central place to update our CI actions. We can also version this repo if we choose.

Examples of how this will be used:

• snapcrafters/discord#168
• snapcrafters/signal-desktop#228
• snapcrafters/mattermost-desktop#93
• snapcrafters/fork-and-rename-me#40

Example runs from my testing repo:

• release to candidate
• version sync
• promotion to stable
• build snap in PR

The README has a good summary, and links to the other actions (and their READMEs).

[jnsgruk]

Example runs from my testing repo:

• release to candidate
• version sync
• promotion to stable
• build snap in PR

[jnsgruk]

So 75d604a fixes our reproducible/revision tracking issue!