Fixes #3415 - Quick reply auto-subscribes

[AndreRl]

Extended from #3342, fixes #3415

[AndreRl]

@euantorano @Eldenroot @effone When you have the time please review if necessary.

[effone]

This is not the desired fix. I placed the method value as dont for a reason. And the reason is this:
https://github.com/mybb/mybb/blob/feature/newreply.php#L835

If you blank out the first value the new reply page will appear with no radio checked instead of first one checked in case of not subscribed:

The desired fix should be keeping the dont as it is and change this line:
https://github.com/mybb/mybb/blob/feature/newreply.php#L382

From
if(!isset($postoptions['subscriptionmethod']))
to
if(!isset($postoptions['subscriptionmethod']) || ($postoptions['subscriptionmethod'] == "dont" && $mybb->get_input('method') == "quickreply"))

[dvz]

This above seems to work fine, but it would introduce internal inconsistency (some functions returning dont while the handler expects an empty string, requiring additional processing). Perhaps the first radio input could be checked when the get_subscription_method() output is empty?

[effone]

In that case additional modifications in var naming and templates are required.
I will try to push some changes in the branch (if required).

[effone]

Alright I have made some changes in the PR. Hope its okay now ...

[dvz]

Looks like the changes of https://github.com/mybb/mybb/pull/3342/files and of this PR still need to be applied to editpost.php.
The template version should be bumped to 1819.