Make forum-post-cooldown use auto-follow-topic's logic

[mybearworld]

Resolves #6926
Probably resolves #7269
Resolves (possibly) #6914 (not sure about this one, I can't seem to replicate that issue even on stable)

Changes

Turn auto-follow-topic into a module, and make forum-post-cooldown use it.

Reason for changes

Improving forum-post-cooldown.

Tests

Tested in Edge.

[WorldLanguages]

This still isn't convincing to me. Shouldn't this check for postError()?

[mybearworld]

postError() is called when setting the success variable.

[WorldLanguages]

success being true does guarantee there wasn't any post errors. However, success being false tells us nothing about whether there was a post error or not. Again, how do we want to define a "non successful post"?

[mybearworld]

A "non-succesful post" is a post that has been attempted to be posted by the user, but instead of it being posted, an error message was shown.

[WorldLanguages]

The variable success is true, if and only if, hasTopic && !postError() is true
That is, !success is true if either hasTopic if false, postError() is true, or both at the same time

To see if there's a non-successful post, you evaluate hasTopic && !success.
But as said above, this is equivalent to hasTopic && (!hasTopic || postError())
Of course, hasTopic cannot be true and false at the same time, so the condition you end up checking is hasTopic && postError()

So I guess your code is equivalent to what I would expect (I would need to check again), but I found it quite confusing!

[mybearworld]

Should I just leave out the success variable in favor of calling postError directly?

[WorldLanguages]

Suggested change

	export const onNonSuccessfulPost = (callback) => {
	if (hasTopic && !success) callback(topicID);
	export const onNonSuccessfulPost = (callback) => {
	if (postError()) callback(topicID);

I guess this would work as you expect?