Update how stale handles exempt items

[johnsudol]

Changes

• New option added remove-stale-from-exempt-items
• Updated how stale interacts with exempt items
• Clarified some of the existing documentation

Context

Originally stale bot would not process exempt items.

In #136, stale bot was updated to remove stale labels from items with exempt labels. This introduced a problem where if a user wanted to manually add a stale label to an item with an exempt label, the stale label would automatically be removed.

To resolve this, I've added a flag remove-stale-from-exempt-items which removes the stale label from an exempt item with recent updates. This comes at the cost of a couple of api calls, so by default this option is set to false.

 When the flag is disabled stale will continue to not process exempt items

TLDR: Stale bot removed stale labels from exempt items no matter what. The new option remove-stale-from-exempt-items allows stale bot to only remove the stale label from exempt issues when they've been recently updated

This PR reverts #136 and adds additional documentation and logging in order to clarify the behaviour of exempt items

Resolves #766

[vanZeben]

Just some language updates that I feel are better, feel free to pick and choose.

Also, we've talked offline about this but just to put the comments in a PR. I personally don't think we should be supporting #136 further and should revert that change entirely.

There are 2 logical ways to infer what the purpose of this action is;

• The first, in Remove stale label when issue is tagged with exempt label after marking stale #136, is that the inference is that the stale label itself is "owned" by this action. With this in mind, exempt means exempt from usage of the label.
• The second is that this action "manages" the stale label on issue's/pr's. With this in mind, exempt means exempt from the action managing that issue/pr.

Personally I feel that the second inference is more appropriate to the spirit of this action. It also means that we don't have to write edge-cases to actively prevent users from labeling their issue's/pr's, which really leads to a confusing user experience (see #766).

This would mean reverting this PR (which is now adding an additional configuration option to handle edge cases), as well as #136 and being more clear on definitions. I'll leave it to discussion from the rest of the team

[JoannaaKL]

So to paraphrase @vanZeben there are two options:

1. Do not allow the issue/pr to be marked as stale, as in #136
2. Do not process the issue/pr with the stale label at all.
   I am also personally in favour of the option number 2 - from my perspective it is more understandable and, as mentioned above, relieves us from supporting all weird edge cases. It'd be also great to explicitly describe it in the docs. (Because if now we're discussing what does extempt mean, imagine someone seeing the plugin for the first time). 😄
   So I'd revert Remove stale label when issue is tagged with exempt label after marking stale #136 too and add more clarification docs.

[johnsudol]

I've updated the PR to be aligned with comments above.

Moving forward this will simplify how stale handles exempt items and will allow us to avoid edge cases such as the flag that was previously mentioned in this PR

[vanZeben]

LGTM