Update page object waiting method for robustness

The `finish_loading` method waits for an element within another
element not to have the css selector `.fa-spinner`, but the ele-
ment used in the `wait_assignees_block_finish_loading` method
always has this class within the element with the following css
selector `.qa-assignee-block`, which makes the `finish_loading`
method not useful for this specific case, but the selector
`.qa-assignee-block .fa-spinner.block-loading` is only shown while
the block is loading, and then such an element is removed from the
DOM.

I noticed the new test that uses the method
`wait_assignees_block_finish_loading` failing on CI, but I could
not reproduce the issue locally, so, this change is a try to fix it.

qa/qa/page/project/issue/show.rb

@@ -159,7 +159,7 @@ module QA
           def wait_assignees_block_finish_loading
             within_element(:assignee_block) do
               wait(reload: false, max: 10, interval: 1) do
-                finished_loading?
+                finished_loading_block?
                 yield
               end
             end