Improve usability of comments editor

The comments editor currently has a number of usability issues:

• The placeholder text of the comment reason dropdown does not work but displays the default placeholder text "Select an option" (this is addressed by https://opencast.jira.com/browse/MH-13051)

• The comment reason dropdown is not width enough to display the comment reason although there would be place of space (-> make it widther)

• The comment reason is a mandatory option, but this is not indicated (-> Add text "Reason:*" to make this obvious. Ensure that the asterisk has the same color/style as in other places in the UI, e.g. Event Details)

• If no comment is available, the UI just displays an empty white area (-> Add text "There are no comments" in this case)

• The list provider default value is not respected (-> Add support for list provider default value)

• If the user presses on "reply" and the on "delete" for a given comments, this yields in an inconsistent dialog (answering to a non-existing comment should not be possible

The same issues appear on those two places:

• Events -> Events -> Event Details -> Comments

• Events -> Events -> Video Editor -> Comments

In the video editor, there is an additional issue:

• The comment tab content is too high when no comments are shown. (-> Align height to another fixed height tab to avoid tab height changes in common cases)