Jetpack: Subscriptions: deactivating module unsets Follow email option

[jeherve]

Steps to reproduce the issue

1. Start from a brand new Jetpack site, with the Subscriptions module on.
2. Go to Settings > Discussion
3. You'll notice that "email me whenever someone follows my blog" is on.
4. Go to Jetpack > Settings > Discussion and deactivate the Subscriptions module.
5. Once you've received confirmation, reactivate it.
6. Go to Settings > Discussion
7. You'll notice that "email me whenever someone follows my blog" is now off.

Ideally the option would remain when you deactivate / reactivate the module.

Reported in p8oabR-sb-p2#comment-3759

[htdat]

@jeherve this is introduced in this PR and this specific code:

• (1) When "Subscriptions" module is activated, the option is set "off".
• (2) When it is deactivated, the option will be removed.

https://github.com/Automattic/jetpack/pull/14239/files#diff-6e4337c8315c90dd70aea03475af1dc1R867-R888

To fix this issue, we just need to remove (2) and change (1)'s behavior to:

• The option is set "off" if the option does not exist.
• Otherwise, do nothing.

I can create a PR. Do you think it's good enough to merge in 8.1 or still need to wait till 8.2?

[jeherve]

Do you think it's good enough to merge in 8.1 or still need to wait till 8.2?

Let's schedule this for 8.2. 👍

[htdat]

I followed up a similar option Someone likes one of my posts from "Likes" module
https://github.com/Automattic/jetpack/blob/8.0/modules/likes.php#L98-L118

I thought it's kinda a standard in handling options in Jetpack. And it also makes sense when deactivating a module, we should clear all of its settings.