Make getting a user by the username case insensitive

What does this MR do?

This merge request makes sure that when creating a new user object searching by username, it does so in a case sensitive manor.

What are the relevant issue numbers?

Closes #38304 (closed)
Closes #27229 (closed)
Possibly solves #47601 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated (After searching, nothing in the docs suggest usernames are sensitive or insensitive),
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

app/finders/user_finder.rb

@@ -7,22 +7,52 @@
 # times we may want to exclude blocked user. By using this finder (and extending
 # it whenever necessary) we can keep this logic in one place.
 class UserFinder
-  attr_reader :params
+  def initialize(username_or_id)
+    @username_or_id = username_or_id
+  end
+
+  # Tries to find a User by id, returning nil if none could be found.
+  def find_by_id
+    User.find_by_id(@username_or_id)
+  end
 
-  def initialize(params)
-    @params = params
+  # Tries to find a User by id, raising a `ActiveRecord::RecordNotFound` if it could
+  # not be found.
+  def find_by_id!
+    User.find(@username_or_id)
   end
 
-  # Tries to find a User, returning nil if none could be found.
-  # rubocop: disable CodeReuse/ActiveRecord
-  def execute
-    User.find_by(id: params[:id])
+  # Tries to find a User by username, returning nil if none could be found.
+  def find_by_username
+    User.find_by_username(@username_or_id)
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 
-  # Tries to find a User, raising a `ActiveRecord::RecordNotFound` if it could
+  # Tries to find a User by username, raising a `ActiveRecord::RecordNotFound` if it could
   # not be found.
-  def execute!
-    User.find(params[:id])
+  def find_by_username!
+    User.find_by_username!(@username_or_id)
+  end
+
+  # Tries to find a User by username or id, returning nil if none could be found.
+  def find_by_id_or_username
+    if input_is_id?
+      find_by_id
+    else
+      find_by_username
+    end
+  end
+
+  # Tries to find a User by username or id, raising a `ActiveRecord::RecordNotFound` if it could
+  # not be found.
+  def find_by_id_or_username!
+    if input_is_id?
+      find_by_id!
+    else
+      find_by_username!
+    end
+  end
+
+  def input_is_id?
+    @username_or_id.is_a?(Numeric) || @username_or_id =~ /^\d+$/
   end
 end