WIP: [Sprint/X](bug): Default hashtags for mature channels

Closes https://gitlab.com/minds/front/issues/915

Mature channels were not automatically being flagged by default as NSFW.

I believe that a newer system has been put in using the user objects nsfw (arr) attribute; based on this I made an assumption that is_mature (bool) is the older systems way of handling this, and wrote a condition for it accordingly, marking the post as the last item other in the list. Without this previously marked channels would not be correctly marked as NSFW - but there was no way to distinguish why their channel is NSFW.

Tested it locally hard-coding

nsfw = [1, 2] 
is_mature = null;

and

nsfw = null
is_mature = true;

and both being null.