Disallow usernames that contain certain words and RegExp (#2200)

* Add `disallowedwords` setting and perform check

Check if any of the words in `disallowedwords` are contained in the username. If
so, return false like other checks.

NOTE: Needs testing for advanced bugs.

* Remove "administrator" from `disallowedwords`

"administrator" contains "admin" anyway, so it is not needed.

* Add error message if username contains a disallowed word

* Add `disallowedregexp` setting and perform check

Check if each expression in `disallowedregexp` exactly matches the username. If
so, return false.

TODO: Add specific error to dialog in `window_main.cpp`.

* Add error message for username matching RegExp

* Fix indentation

* Compile `disallowedregexp` into a QList upon initialization

Reduces system load with each registration request.

* Clean up `isUsernameValid` function

* Fix indentation

* Add backwards compatibility to client

Client can accept either 7 or 9 rules to maintain compatibility with older
versions of server.

* Add examples and warnings to `servatrice.ini`

servatrice/servatrice.ini.example

@@ -107,6 +107,18 @@ allowedpunctuation=_.-
 ; If a username can begin with punctuation defined in allowedpunctuation
 allowpunctuationprefix=false
 
+; Disallow usernames containing these words. This list is comma seperated, e.g.
+; "admin,user,name"
+disallowedwords="admin"
+
+; Disallow usernames matching these regular expressions. This list is comma
+; separated, e.g. "\\w+\\d+,\\d{2}user", hence you cannot use commas in your
+; expressions. Backslashes must be escaped, so `\w+\d+` becomes `\\w+\\d+`.
+; WARNING: Complex expressions can be harmful to performance. Please make sure
+;          your expressions are considered well formed. See this page for info:
+;          http://www.regular-expressions.info/catastrophic.html
+disallowedregexp=""
+
 [registration]
 
 ; Servatrice can process registration requests to add new users on the fly.