Add fix for reset-password and new user creation support with Solid Security to compatibility layer

[jazzsequence]

see pantheon-systems/documentation#9388

Solid Security (formerly iThemes) has some additional tweaks that have been known internally in CSE but not surfaced to the Plugins With Known Issues docs page until recently. The issue derives from a URL query parameter being stripped on redirects that can potentially lock users out of their site. Solutions are in the PR.

The first step is adding a filter, which I believe can easily be done inside the mu-plugin's compatibility layer. The second step requires code to be added to wp-config.php. I think that this could be done elsewhere (e.g. inside a hook) and therefore inside the mu-plugin, but this needs to be tested.

The plugin is already listed as partially compatible in the plugin, so, while we could fix this one thing, we wouldn't be fixing everything. Additionally, while the partial compatibility is registered, there are no existing fixes for anything relating to this plugin -- the applied fix(es) would be new.

Done When

• Solutions have been tested inside the mu-plugin
• A new update of the mu-plugin is released

Notes

This is probably a fairly low-ish priority given that the plugin itself is already registered as "Partially Compatible", but applying the fix would be a boon to anyone using the plugin.

https://wordpress.org/plugins/better-wp-security/