Preload and cache "insert link" forms

[chillu]

Acceptance Criteria

• As a CMS author, I can insert links without breaking my writing flow due to sluggish UIs
• All "insert link" forms are available without a loading delay in the browser after first load
• The "page" dropdown refreshes whenever the "insert link to page" form is opened
• The "insert anchor" form automatically has the current page set when the form is opened
• The anchor list refreshes whenever the "insert anchor" form is opened
• The caches are cleared when the CMS language is changed
• The caches can be cleared by the user through default browser mechanisms

Notes

• This is one of the most common user actions in the CMS, so detail optimisations are warranted
• Every form schema based form should be able to opt-in to this logic
• HTTP-based caching rules would allow easy cache clearing: Either through expiry times, or through a hard browser refresh by the user.
• Technically different users could get different form schema responses - we should tie these caches to various environment criteria (UserID, SubsiteID)
• This is a partial performance regression from SS3: The "insert link" form only loaded the first time and persists in the DOM until the browser is refreshed
• Follow up from https://github.com/silverstripe/silverstripe-framework/issues/5947