feat(admin-plugins): display inline notification before updating

[mathetos]

Issue Overview

When Give has a major update, it would be excellent to give our users additional warnings BEFORE they update. This tutorial walks through how that can be done by creating inline update messages:
https://wisdomplugin.com/add-inline-plugin-update-message

Being able to say "Hey, version X.XX has database updates. Please backup your site first. And read all about the awesome features on this blog post" would be really useful.

[DevinWalker]

I'm not seeing the warning display within the plugin update message upon testing:

@Sidsector9 I'm reopening. Can you recheck this and make sure I've configured the readme properly: https://github.com/WordImpress/Give/blob/master/readme.txt#L388

[mathetos]

I meant to reopen this yesterday too. Also couldn't get it to show correctly.

[ravinderk]

@mathetos I found the issue.

@Sidsector9 using hard-coded base directory name instead of dynamic.
in_plugin_update_message-Give/give.php (current) --> in_plugin_update_message-{GIVE_PLUGIN_BASENAME} (recommanded)

I found another issue too about logic which I will discuss with @Sidsector9.

for ref: https://github.com/WordImpress/Give/blob/master/includes/admin/plugins.php#L280

[ravinderk]

This issue has been resolved but I am reopening this to test with few examples. I will close this if it will work fine for me.

[ravinderk]

@mathetos @DevinWalker working fine for me.