DEV: Reuse can_invite_to_forum? in can_invite_to? (#14392)

This commit resolves refactors can_invite_to? to use
can_invite_to_forum? for checking the site-wide permissions and then
perform topic specific checkups.

Similarly, can_invite_to? is always used with a topic object and this is
now enforced.

There was another problem before when `must_approve_users` site setting
was not checked when inviting users to forum, but was checked when
inviting to a topic.

Another minor security issue was that group owners could invite to
group topics even if they did not have the minimum trust level to do
it.

spec/requests/topics_controller_spec.rb

@@ -3868,7 +3868,7 @@ RSpec.describe TopicsController do
         fab!(:topic) { Fabricate(:topic, user: user) }
 
         it 'should return the right response' do
-          user.update!(trust_level: TrustLevel[2])
+          user.update!(trust_level: SiteSetting.min_trust_level_to_allow_invite)
 
           expect do
             post "/t/#{topic.id}/invite.json", params: {
@@ -3891,6 +3891,10 @@ RSpec.describe TopicsController do
 
         let!(:recipient) { 'jake@adventuretime.ooo' }
 
+        before do
+          user.update!(trust_level: SiteSetting.min_trust_level_to_allow_invite)
+        end
+
         it "should attach group to the invite" do
           post "/t/#{group_private_topic.id}/invite.json", params: {
             user: recipient,