Per the tracker issue:
> In 3.1 a youtube profilefield was added in which we can insert our youtube
> username to link to it.... There is just one major problem though...
>
> New youtube members no longer get an username. So I think we should decide on
> providing different ways to access their profile.
This PR will allow users to use any valid YouTube-domain URL, which allows for
all 3 formats (`/channel/...`, `/c/...`, and the now-legacy `/user/...`).
Per [YouTube's docs](https://support.google.com/youtube/answer/6180214?hl=en):
> ## Channel URL (ID-based)
> Example: youtube.com/channel/UCUZHFZ9jIKrLroW8LcyJEQQ
>
> This is the standard URL that YouTube channels use.
>
> ...
>
> ## Custom URL
> Example: youtube.com/c/YouTubeCreators
>
> A custom URL is a shorter, easy-to-remember URL that you can share with your
> audience.
>
> ...
>
> ## Legacy username URL
> Example: youtube.com/user/YouTube
>
> Depending on when your channel was created, it may have a username. Usernames
> are no longer required for channels today, but you can still use this URL to
> direct to your channel — even if your channel name has changed since you chose
> your username. Existing usernames can't be changed.
PHPBB3-13859
The code present in the prosilver style template causes the wrong
checkbox to be displayed in the post editor if the combination
`f_delete`:true and `f_softdelete`:false is active in the user
context. With this combination, this effectively prevents a forum
post from being deleted via the editor options.
Corrected the relevant section and switched to Twig syntax
PHPBB3-16990
Fixes [DBAL function sql_nextid - name is
misleading](https://tracker.phpbb.com/browse/PHPBB3-8071).
Per the tracker issue:
> This function gets the ID of the last inserted row, not the ID of the next row
to be inserted (which is difficult due to different autoincrement steps and also
lends itself nicely to race conditions).
@iwisdom replied that the naming is consistent with how PHP itself names this
functionality, but the link they posted doesn't support that (I guess this is
referring to a much older version of PHP? The comment is from 2009).
PHPBB3-8071
