Align @-mentions and x-post messaging with WPAndroid

Expected behavior

Improved @-mention and xpost messaging.

Actual behavior

• On WPAndroid, there's a "No matching users." message shown when what the user types doesn't match any results from the @-mention suggestion list (iOS doesn't show anything at the moment) — the same applies to xposts where a "No matching crosspost." message is shown on WPAndroid but not on WPiOS
• If I then go ahead and hit the keyboard Done button while the "No matching users." message is shown, I see a "@xyz is not a valid user" toast message (same applies to xposts where I see a "+xyz is not a valid crosspost")
• When the user is offline and triggers either type of suggestions while there is no cached data available, WPAndroid shows an appropriate message while WPiOS does not
• When a user triggers the suggestion UI and the network result indicates there are no suggestions available, WPAndroid shows an appropriate message while WPiOS does not

Tested on iPhone 11, iOS 14.0, WPiOS 15.9

Compare the WPAndroid and WPiOS behavior for @-mention and x-post suggestions, including no matches, offline results, and network responses with no suggestions. Reproduce the missing messages and invalid-user or invalid-crosspost toasts on iOS, then verify that its suggestion UI matches the Android behavior.