Allow a plugin to update phplist

[bramley]

Description

Identify whether a plugin is able to update the phplist code, and link to that plugins's update page when a new release is available

Related Issue

939

Screenshots (if appropriate):

[bramley]

To test this, you will need to patch the AddonsPlugin.php file

class AddonsPlugin extends phplistPlugin implements Updater

I can update that plugin only after this change has been released.

[michield]

That forces the updater plugin to always use "update" as the page to run the update. I guess that's fine, but could be made more abstract.

[phpListDockerBot]

This pull request has been mentioned on phpList Discuss. There might be relevant details there:

https://discuss.phplist.org/t/3-6-13-release-candidate-ready-for-testing/8882/1

[phpListDockerBot]

This pull request has been mentioned on phpList Discuss. There might be relevant details there:

https://discuss.phplist.org/t/phplist-3-6-13-has-been-released/8915/1